WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
Series: WAMU.04S1
Distribution Date: 12/27/2004

Class	Cusip	Orig Prin Bal	Begin Prin Bal	Int Rate Accrued	Int Rate Paid	Int Dist	Prin Dist	Int Shortfall	PPIS	Uncomp Int Shortfall	Prin Loss	Deferred Interest	Int Reimbursed	Unsched Int	Prin Reimbursed	Subsequent Recoveries	Total Dist	Ending Class Bal
1A1	92922FLR7	$99,827,849.00	$95,204,480.45	4.0500000000%	4.0500000000%	$321,315.12	$770,727.38	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$1,092,042.50	$94,433,753.07
1A2	92922FLS5	$13,000,000.00	$13,000,000.00	4.0500000000%	4.0500000000%	$43,875.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$43,875.00	$13,000,000.00
1A3	92922FLT3	$65,440,151.00	$62,758,597.32	2.5875000000%	2.5875000000%	$135,323.23	$447,021.87	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$582,345.10	$62,311,575.45
1A4	92922FLU0	$65,440,151.00	$62,758,597.32	5.4125000000%	5.4125000000%	$283,067.42	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$283,067.42	$62,311,575.45
1A5	92922FLV8	$56,668,000.00	$44,471,239.94	5.5000000000%	5.5000000000%	$203,826.52	$3,135,134.35	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$3,338,960.87	$41,336,105.59
1A6	92922FLW6	$7,747,663.00	$7,747,663.00	5.5000000000%	5.5000000000%	$35,510.12	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$35,510.12	$7,747,663.00
1A7	92922FLX4	$19,800,000.00	$0.00	0.0000000000%	0.0000000000%	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00
1A8	92922FLY2	$7,200,000.00	$0.00	0.0000000000%	0.0000000000%	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00
1A9	92922FLZ9	$15,000,000.00	$15,000,000.00	5.5000000000%	5.5000000000%	$68,750.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$68,750.00	$15,000,000.00
1A10	92922FMA3	$20,000,000.00	$20,000,000.00	5.5000000000%	5.5000000000%	$91,666.67	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$91,666.67	$20,000,000.00
1A11	92922FMB1	$600,000.00	$600,000.00	5.5000000000%	5.5000000000%	$2,750.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$2,750.00	$600,000.00
1A12	92922FMC9	$11,031,529.00	$11,031,529.00	5.5000000000%	5.5000000000%	$50,561.17	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$50,561.17	$11,031,529.00
2A1	92922FMD7	$45,369,689.00	$34,499,375.16	2.7375000000%	2.7375000000%	$78,701.70	$1,337,979.24	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$1,416,680.94	$33,161,395.92
2A2	92922FME5	$45,369,689.00	$34,499,375.16	5.2625000000%	5.2625000000%	$151,294.13	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$151,294.13	$33,161,395.92
P	92922FMF2	$2,438,018.62	$2,391,692.94	0.0000000000%	0.0000000000%	$0.00	$3,232.67	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$3,232.67	$2,388,460.27
B1	92922FMG0	$5,256,000.00	$5,206,623.18	5.8137518408%	5.8137518408%	$25,225.01	$5,723.83	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$30,948.84	$5,200,899.35
B2	92922FMH8	$2,252,000.00	$2,230,843.87	5.8137518408%	5.8137518408%	$10,807.98	$2,452.45	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$13,260.42	$2,228,391.42
B3	92922FMJ4	$1,502,000.00	$1,487,889.65	5.8137518408%	5.8137518408%	$7,208.52	$1,635.69	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$8,844.21	$1,486,253.96
B4	92922FLN6	$750,000.00	$742,954.22	5.8137518408%	5.8137518408%	$3,599.46	$816.76	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$4,416.22	$742,137.46
B5	92922FLP1	$751,000.00	$743,944.82	5.8137518408%	5.8137518408%	$3,604.26	$817.85	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$4,422.10	$743,126.97
B6	92922FLQ9	$751,346.35	$744,287.92	5.8137518408%	5.8137518408%	$3,605.92	$818.22	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$4,424.14	$743,469.70
R	92922FMK1	$100.00	$0.00	0.0000000000%	0.0000000000%	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00
R_SYN	XXXXXXXXX	$0.00	$0.00	0.0000000000%	0.0000000000%	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00
Total		$375,385,345.97	$317,861,121.47			$1,520,692.24	$5,706,360.29	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$7,227,052.53	$312,154,761.18

WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
Series: WAMU.04S1
Distribution Date: 12/27/2004

Class	Orig Face	Beg Bal	Prin Loss	Prin Reimbursed	Subsequent Recoveries	Deferred Interest	Total Prin	End Cert Pct	End Cert Bal
1A1	$99,827,849.00	$95,204,480.45	$0.00	$0.00	$0.00	$0.00	$770,727.38	0.9459660207	$94,433,753.07
1A2	$13,000,000.00	$13,000,000.00	$0.00	$0.00	$0.00	$0.00	$0.00	1.0000000000	$13,000,000.00
1A3	$65,440,151.00	$62,758,597.32	$0.00	$0.00	$0.00	$0.00	$447,021.87	0.9521918042	$62,311,575.45
1A4	$65,440,151.00	$62,758,597.32	$0.00	$0.00	$0.00	$0.00	$0.00	0.9521918042	$62,311,575.45
1A5	$56,668,000.00	$44,471,239.94	$0.00	$0.00	$0.00	$0.00	$3,135,134.35	0.7294435235	$41,336,105.59
1A6	$7,747,663.00	$7,747,663.00	$0.00	$0.00	$0.00	$0.00	$0.00	1.0000000000	$7,747,663.00
1A7	$19,800,000.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	0.0000000000	$0.00
1A8	$7,200,000.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	0.0000000000	$0.00
1A9	$15,000,000.00	$15,000,000.00	$0.00	$0.00	$0.00	$0.00	$0.00	1.0000000000	$15,000,000.00
1A10	$20,000,000.00	$20,000,000.00	$0.00	$0.00	$0.00	$0.00	$0.00	1.0000000000	$20,000,000.00
1A11	$600,000.00	$600,000.00	$0.00	$0.00	$0.00	$0.00	$0.00	1.0000000000	$600,000.00
1A12	$11,031,529.00	$11,031,529.00	$0.00	$0.00	$0.00	$0.00	$0.00	1.0000000000	$11,031,529.00
2A1	$45,369,689.00	$34,499,375.16	$0.00	$0.00	$0.00	$0.00	$1,337,979.24	0.7309152135	$33,161,395.92
2A2	$45,369,689.00	$34,499,375.16	$0.00	$0.00	$0.00	$0.00	$0.00	0.7309152135	$33,161,395.92
P	$2,438,018.62	$2,391,692.94	$0.00	$0.00	$0.00	$0.00	$3,232.67	0.9796726942	$2,388,460.27
B1	$5,256,000.00	$5,206,623.18	$0.00	$0.00	$0.00	$0.00	$5,723.83	0.9895166191	$5,200,899.35
B2	$2,252,000.00	$2,230,843.87	$0.00	$0.00	$0.00	$0.00	$2,452.45	0.9895166163	$2,228,391.42
B3	$1,502,000.00	$1,487,889.65	$0.00	$0.00	$0.00	$0.00	$1,635.69	0.9895166178	$1,486,253.96
B4	$750,000.00	$742,954.22	$0.00	$0.00	$0.00	$0.00	$816.76	0.9895166133	$742,137.46
B5	$751,000.00	$743,944.82	$0.00	$0.00	$0.00	$0.00	$817.85	0.9895166045	$743,126.97
B6	$751,346.35	$744,287.92	$0.00	$0.00	$0.00	$0.00	$818.22	0.9895166191	$743,469.70
R	$100.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	0.0000000000	$0.00
R_SYN	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	0.0000000000	$0.00
Total	$375,385,345.97	$317,861,121.47	$0.00	$0.00	$0.00	$0.00	$5,706,360.29	0.8315581962	$312,154,761.18

WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
Series: WAMU.04S1
Distribution Date: 12/27/2004
PER $1000 DENOMINATION
Class	Orig Face	Beg Bal	Prin Loss	Prin Reimbursed	Subsequent Recoveries	Deferred Interest	Total Prin	End Cert Pct	End Cert Bal
1A1	$99,827,849.00	953.68658549	0.00000000	0.00000000	0.00000000	0.00000000	7.72056483	0.9459660207	945.96602066
1A2	$13,000,000.00	1,000.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	1.0000000000	1,000.00000000
1A3	$65,440,151.00	959.02280727	0.00000000	0.00000000	0.00000000	0.00000000	6.83100303	0.9521918042	952.19180423
1A4	$65,440,151.00	959.02280727	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.9521918042	952.19180423
1A5	$56,668,000.00	784.76812204	0.00000000	0.00000000	0.00000000	0.00000000	55.32459854	0.7294435235	729.44352351
1A6	$7,747,663.00	1,000.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	1.0000000000	1,000.00000000
1A7	$19,800,000.00	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.0000000000	0.00000000
1A8	$7,200,000.00	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.0000000000	0.00000000
1A9	$15,000,000.00	1,000.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	1.0000000000	1,000.00000000
1A10	$20,000,000.00	1,000.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	1.0000000000	1,000.00000000
1A11	$600,000.00	1,000.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	1.0000000000	1,000.00000000
1A12	$11,031,529.00	1,000.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	1.0000000000	1,000.00000000
2A1	$45,369,689.00	760.40581103	0.00000000	0.00000000	0.00000000	0.00000000	29.49059757	0.7309152135	730.91521346
2A2	$45,369,689.00	760.40581103	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.7309152135	730.91521346
P	$2,438,018.62	980.99863569	0.00000000	0.00000000	0.00000000	0.00000000	1.32594147	0.9796726942	979.67269421
B1	$5,256,000.00	990.60562785	0.00000000	0.00000000	0.00000000	0.00000000	1.08900875	0.9895166191	989.51661910
B2	$2,252,000.00	990.60562611	0.00000000	0.00000000	0.00000000	0.00000000	1.08900977	0.9895166163	989.51661634
B3	$1,502,000.00	990.60562583	0.00000000	0.00000000	0.00000000	0.00000000	1.08900799	0.9895166178	989.51661784
B4	$750,000.00	990.60562667	0.00000000	0.00000000	0.00000000	0.00000000	1.08901333	0.9895166133	989.51661333
B5	$751,000.00	990.60561917	0.00000000	0.00000000	0.00000000	0.00000000	1.08901465	0.9895166045	989.51660453
B6	$751,346.35	990.60562416	0.00000000	0.00000000	0.00000000	0.00000000	1.08900509	0.9895166191	989.51661907
R	$100.00	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.0000000000	0.00000000
R_SYN	$0.00	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.0000000000	0.00000000

WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
Series: WAMU.04S1
Distribution Date: 12/27/2004

Class	Orig Face	Int Rate Accrued	Int Rate Paid	Beg Cert Prin/Not Bal	Current Int Accrued	Int Shortfall	PPIS	Uncompensated Int Shortfall	Unscheduled Int	Int Reimbursed	Total Int Dist
1A1	$99,827,849.00	4.0500000000%	4.0500000000%	$95,204,480.45	$321,315.12	$0.00	$0.00	$0.00	$0.00	$0.00	$321,315.12
1A2	$13,000,000.00	4.0500000000%	4.0500000000%	$13,000,000.00	$43,875.00	$0.00	$0.00	$0.00	$0.00	$0.00	$43,875.00
1A3	$65,440,151.00	2.5875000000%	2.5875000000%	$62,758,597.32	$135,323.23	$0.00	$0.00	$0.00	$0.00	$0.00	$135,323.23
1A4	$65,440,151.00	5.4125000000%	5.4125000000%	$62,758,597.32	$283,067.42	$0.00	$0.00	$0.00	$0.00	$0.00	$283,067.42
1A5	$56,668,000.00	5.5000000000%	5.5000000000%	$44,471,239.94	$203,826.52	$0.00	$0.00	$0.00	$0.00	$0.00	$203,826.52
1A6	$7,747,663.00	5.5000000000%	5.5000000000%	$7,747,663.00	$35,510.12	$0.00	$0.00	$0.00	$0.00	$0.00	$35,510.12
1A7	$19,800,000.00	0.0000000000%	0.0000000000%	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00
1A8	$7,200,000.00	0.0000000000%	0.0000000000%	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00
1A9	$15,000,000.00	5.5000000000%	5.5000000000%	$15,000,000.00	$68,750.00	$0.00	$0.00	$0.00	$0.00	$0.00	$68,750.00
1A10	$20,000,000.00	5.5000000000%	5.5000000000%	$20,000,000.00	$91,666.67	$0.00	$0.00	$0.00	$0.00	$0.00	$91,666.67
1A11	$600,000.00	5.5000000000%	5.5000000000%	$600,000.00	$2,750.00	$0.00	$0.00	$0.00	$0.00	$0.00	$2,750.00
1A12	$11,031,529.00	5.5000000000%	5.5000000000%	$11,031,529.00	$50,561.17	$0.00	$0.00	$0.00	$0.00	$0.00	$50,561.17
2A1	$45,369,689.00	2.7375000000%	2.7375000000%	$34,499,375.16	$78,701.70	$0.00	$0.00	$0.00	$0.00	$0.00	$78,701.70
2A2	$45,369,689.00	5.2625000000%	5.2625000000%	$34,499,375.16	$151,294.13	$0.00	$0.00	$0.00	$0.00	$0.00	$151,294.13
P	$2,438,018.62	0.0000000000%	0.0000000000%	$2,391,692.94	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00
B1	$5,256,000.00	5.8137518408%	5.8137518408%	$5,206,623.18	$25,225.01	$0.00	$0.00	$0.00	$0.00	$0.00	$25,225.01
B2	$2,252,000.00	5.8137518408%	5.8137518408%	$2,230,843.87	$10,807.98	$0.00	$0.00	$0.00	$0.00	$0.00	$10,807.98
B3	$1,502,000.00	5.8137518408%	5.8137518408%	$1,487,889.65	$7,208.52	$0.00	$0.00	$0.00	$0.00	$0.00	$7,208.52
B4	$750,000.00	5.8137518408%	5.8137518408%	$742,954.22	$3,599.46	$0.00	$0.00	$0.00	$0.00	$0.00	$3,599.46
B5	$751,000.00	5.8137518408%	5.8137518408%	$743,944.82	$3,604.26	$0.00	$0.00	$0.00	$0.00	$0.00	$3,604.26
B6	$751,346.35	5.8137518408%	5.8137518408%	$744,287.92	$3,605.92	$0.00	$0.00	$0.00	$0.00	$0.00	$3,605.92
R	$100.00	0.0000000000%	0.0000000000%	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00
R_SYN	$0.00	0.0000000000%	0.0000000000%	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00	$0.00
Total	$375,385,345.97			$317,861,121.47	$1,520,692.24	$0.00	$0.00	$0.00	$0.00	$0.00	$1,520,692.24

WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
Series: WAMU.04S1
Distribution Date: 12/27/2004
PER $1000 DENOMINATION
Class	Orig Face	Int Rate Accrued	Int Rate Paid	Beg Cert Prin/Not Bal	Curent Int Accrued	Int Shortfall	PPIS	Uncompensated Int Shortfall	Unscheduled Int	Int Reimbursed	Total Int Dist
1A1	$99,827,849.00	4.0500000000%	4.0500000000%	953.68658549	3.21869221	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	3.21869221
1A2	$13,000,000.00	4.0500000000%	4.0500000000%	1,000.00000000	3.37500000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	3.37500000
1A3	$65,440,151.00	2.5875000000%	2.5875000000%	959.02280727	2.06789300	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	2.06789300
1A4	$65,440,151.00	5.4125000000%	5.4125000000%	959.02280727	4.32559240	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	4.32559240
1A5	$56,668,000.00	5.5000000000%	5.5000000000%	784.76812204	3.59685396	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	3.59685396
1A6	$7,747,663.00	5.5000000000%	5.5000000000%	1,000.00000000	4.58333306	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	4.58333306
1A7	$19,800,000.00	0.0000000000%	0.0000000000%	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000
1A8	$7,200,000.00	0.0000000000%	0.0000000000%	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000
1A9	$15,000,000.00	5.5000000000%	5.5000000000%	1,000.00000000	4.58333333	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	4.58333333
1A10	$20,000,000.00	5.5000000000%	5.5000000000%	1,000.00000000	4.58333350	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	4.58333350
1A11	$600,000.00	5.5000000000%	5.5000000000%	1,000.00000000	4.58333333	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	4.58333333
1A12	$11,031,529.00	5.5000000000%	5.5000000000%	1,000.00000000	4.58333292	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	4.58333292
2A1	$45,369,689.00	2.7375000000%	2.7375000000%	760.40581103	1.73467577	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	1.73467577
2A2	$45,369,689.00	5.2625000000%	5.2625000000%	760.40581103	3.33469621	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	3.33469621
P	$2,438,018.62	0.0000000000%	0.0000000000%	980.99863569	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000
B1	$5,256,000.00	5.8137518408%	5.8137518408%	990.60562785	4.79927892	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	4.79927892
B2	$2,252,000.00	5.8137518408%	5.8137518408%	990.60562611	4.79928064	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	4.79928064
B3	$1,502,000.00	5.8137518408%	5.8137518408%	990.60562583	4.79928096	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	4.79928096
B4	$750,000.00	5.8137518408%	5.8137518408%	990.60562667	4.79928000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	4.79928000
B5	$751,000.00	5.8137518408%	5.8137518408%	990.60561917	4.79928096	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	4.79928096
B6	$751,346.35	5.8137518408%	5.8137518408%	990.60562416	4.79927799	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	4.79927799
R	$100.00	0.0000000000%	0.0000000000%	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000
R_SYN	$0.00	0.0000000000%	0.0000000000%	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000	0.00000000

WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
MASTER SERVICING
MORTGAGE PASS-THROUGH CERTIFICATES
12/2004 DISTRIBUTION REPORT
SERIES: WAMU 04S1
WEIGHTED AVERAGE PC RATE: 5.74097%
WEIGHTED AVERAGE MATURITY: 345.65325
WEIGHTED AVERAGE LOAN AGE: 11.03894
POOL NUMBER: 2343
ISSUE DATE: 02/24/2004
CERTIFICATE BALANCE AT ISSUE: $375,385,346.35
	TOTAL NUMBER OF MORTAGES	CERTIFICATE ACCOUNT ACTIVITY (@ PCRATE)	CERTIFICATE BALANCE OUTSTANDING
BALANCES FROM LAST FISCAL MONTH-END:	659		$317,861,121.85
PRINCIPAL POOL COLLECTION(S):
Scheduled Principal Collection Due Current Month		$350,030.47
Unscheduled Principal Collection/Reversals		$38,644.47
Liquidations-in-full	11	$5,317,685.35
Subsequent Recoveries		$0.00
Net principal Distributed		$5,706,360.29	-$5,706,360.29

CAPITAL LOSS (PRINCIPAL WRITTEN OFF):			$0.00

BALANCE CURRENT FISCAL MONTH-END:	648		$312,154,761.56

SCHEDULED INTEREST AT MORTGAGE RATE:		$1,586,913.12

UNSCHEDULED INTEREST AT MORTGAGE RATE:
Unscheduled Interest Collection/Reversals		$0.00
Interest Uncollected on Liquidation		$0.00
Interest Uncollected on Non-Earning Assets		$0.00
Interest Uncollected on Prepayment		$0.00
Uncompensated Uncollected Interest		$0.00
Net Unscheduled Interest Distributed		$0.00

OTHER:
Loan Conversion Fees		$0.00
Expense Reimbursements		$0.00
Gain on Liquidations		$0.00
Hazard Insurance Premium Refunds		$0.00
Recoveries Carried Forward		$0.00
Net Other Distributions		$0.00

WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
MASTER SERVICING
MORTGAGE PASS-THROUGH CERTIFICATES
12/2004 DISTRIBUTION REPORT
	TOTAL NUMBER OF MORTAGES	CERTIFICATE ACCOUNT ACTIVITY (@ PCRATE)	CERTIFICATE BALANCE OUTSTANDING
SCHEDULED SERVICING FEE EXPENSES:		$66,220.75

UNSCHEDULED SERVICING FEES:
Unscheduled Service Fee Collections/Reversals		$0.00
Servicing Fees Uncollected on Liquidation		$0.00
Servicing Fees Uncollected/Non-Earning Assets		$0.00
Net Unscheduled Service Fees Distributed		$0.00

MISCELLANEOUS EXPENSES:		$0.00

NET FUNDS DISTRIBUTED:		$7,227,052.66

WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
MASTER SERVICING
MORTGAGE PASS-THROUGH CERTIFICATES
12/2004 DISTRIBUTION REPORT
SERIES: WAMU 04S1
WEIGHTED AVERAGE PC RATE: 5.74097%
WEIGHTED AVERAGE MATURITY: 345.65325
WEIGHTED AVERAGE LOAN AGE: 11.03894
POOL NUMBER: ALL
ISSUE DATE: 02/24/2004
CERTIFICATE BALANCE AT ISSUE: $375,385,346.35
	TOTAL NUMBER OF MORTAGES	CERTIFICATE ACCOUNT ACTIVITY (@ PCRATE)	CERTIFICATE BALANCE OUTSTANDING
BALANCES FROM LAST FISCAL MONTH-END:	659		$317,861,121.85
PRINCIPAL POOL COLLECTION(S):
Scheduled Principal Collection Due Current Month		$350,030.47
Unscheduled Principal Collection/Reversals		$38,644.47
Liquidations-in-full	11	$5,317,685.35
Subsequent Recoveries		$0.00
Net principal Distributed		$5,706,360.29	-$5,706,360.29

CAPITAL LOSS (PRINCIPAL WRITTEN OFF):			$0.00

BALANCE CURRENT FISCAL MONTH-END:	648		$312,154,761.56

SCHEDULED INTEREST AT MORTGAGE RATE:		$1,586,913.12

UNSCHEDULED INTEREST AT MORTGAGE RATE:
Unscheduled Interest Collection/Reversals		$0.00
Interest Uncollected on Liquidation		$0.00
Interest Uncollected on Non-Earning Assets		$0.00
Interest Uncollected on Prepayment		$0.00
Uncompensated Uncollected Interest		$0.00
Net Unscheduled Interest Distributed		$0.00

OTHER:
Loan Conversion Fees		$0.00
Expense Reimbursements		$0.00
Gain on Liquidations		$0.00
Hazard Insurance Premium Refunds		$0.00
Recoveries Carried Forward		$0.00
Net Other Distributions		$0.00

WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
MASTER SERVICING
MORTGAGE PASS-THROUGH CERTIFICATES
12/2004 DISTRIBUTION REPORT
	TOTAL NUMBER OF MORTAGES	CERTIFICATE ACCOUNT ACTIVITY (@ PCRATE)	CERTIFICATE BALANCE OUTSTANDING
SCHEDULED SERVICING FEE EXPENSES:		$66,220.75

UNSCHEDULED SERVICING FEES:
Unscheduled Service Fee Collections/Reversals		$0.00
Servicing Fees Uncollected on Liquidation		$0.00
Servicing Fees Uncollected/Non-Earning Assets		$0.00
Net Unscheduled Service Fees Distributed		$0.00

MISCELLANEOUS EXPENSES:		$0.00

NET FUNDS DISTRIBUTED:		$7,227,052.66

WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
Series: WAMU.04S1
Distribution Date: 12/27/2004

Distribution by Group
Pool ID		Principal		Interest		Total
Pool - 2343		$5,706,360.29		$1,520,692.37		$7,227,052.66
Aggregate		$5,706,360.29		$1,520,692.37		$7,227,052.66

Number of Loans and Aggregate Principal Balance By Group
Pool ID	Beg Bal	Beg Count	Paid Off / Liq Count	Rem Count	Sched Prin End Bal	Sched Prin	Prin PrePay / Liq	Unsched Prin / Rev	Total Prin
Pool - 2343	$317,861,121.85	659	11	648	$312,154,761.56	$350,030.47	$5,317,685.35	$38,644.47	$5,706,360.29
Aggregate	$317,861,121.85	659	11	648	$312,154,761.56	$350,030.47	$5,317,685.35	$38,644.47	$5,706,360.29

Delinquency Information by Group
Pool ID	1 Month Count	Sched End Balance	2 Months Count	Sched End Balance	3 Months Count	Sched End Balance	4 Months Count	Sched End Balance
Pool - 2343	4	$1,863,389.83	0	$0.00	0	$0.00	0	$0.00
Aggregate	4	$1,863,389.83	0	$0.00	0	$0.00	0	$0.00

Foreclosure and REO Information by Group
Pool ID		Foreclosure Count		Sched End Balance		REO Count		Sched End Balance
Pool - 2343		1		$345,296.91		0		$0.00
Aggregate		1		$345,296.91		0		$0.00

Aggregate Loss Amounts by Group
Pool ID			Count			Loss Amount
Pool - 2343			0			$0.00
Aggregate			0			$0.00

WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
Series: WAMU.04S1
Distribution Date: 12/27/2004

INSURANCE RESERVES - Group 1
Insurance Type	Original Balance	Claims In Progress	Claims Paid	Coverage Adjustments	Coverage Remaining
MPI	$0.00	$0.00	$0.00	$0.00	$0.00
Special Hazard	$3,753,853.00	$0.00	$0.00	$0.00	$3,753,853.00
Bankruptcy Bond
Single-Units	$100,000.00	$0.00	$0.00	$0.00	$100,000.00
Multi-Units	$0.00	$0.00	$0.00	$0.00	$0.00
Mortgage Repurchase	$7,507,706.00	$0.00	$0.00	$0.00	$7,507,706.00

INSURANCE RESERVES - Aggregate
Insurance Type	Original Balance	Claims In Progress	Claims Paid	Coverage Adjustments	Coverage Remaining
MPI	$0.00	$0.00	$0.00	$0.00	$0.00
Special Hazard	$3,753,853.00	$0.00	$0.00	$0.00	$3,753,853.00
Bankruptcy Bond
Single-Units	$100,000.00	$0.00	$0.00	$0.00	$100,000.00
Multi-Units	$0.00	$0.00	$0.00	$0.00	$0.00
Mortgage Repurchase	$7,507,706.00	$0.00	$0.00	$0.00	$7,507,706.00

WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
Series: WAMU.04S1
Distribution Date: 12/27/2004

The Group B Certificates provide, to the limited extent described in the Prospectus Supplement, credit support, as well as special hazard, bankruptcy, and fraud coverage to certain Certificates. The "Prospectus Supplement" is that certain Prospectus Supplement, dated February 20,2004 to that certain Prospectus Supplement, dated February 10,2004, to which certain certificates were offered. The special hazard, bankruptcy, and Fraud coverage (collectively, "Credit Enhancements") provided by the B certificates provide coverage to the Loans, subject to the conditions and limitations to payment specified thereunder.



The Class Principal Balances of each Class of the Class B1,B2,B3,B4,B5,B6 Certificatesimmediately after the principal and interest distribution on 200412 are as follows:

Class			Class Principal Balance
B1			$5,200,899.35
B2			$2,228,391.42
B3			$1,486,253.96
B4			$742,137.46
B5			$743,126.97
B6			$743,469.70
Total			$11,144,278.87
Capitalized items used but not defined herein have the meanings ascribed to them in the Prospectus Supplement.
The amount of the special hazard coverage, bankruptcy coverage, and fraud coverage, as of the above referenced distribution date, is $3,753,853.00, $100,000.00, $7,507,706.00 respectively, and such coverages are referred to in the table above under the column "Insurance Type" as "special hazard," "bankruptcy bond," and "mortgage repurchase," respectively. However, no policy of insurance exists for any such coverage, notwithstanding the reference to "insurance."

WAMU MORTGAGE PASS THROUGH CERTS SERIES 2004-S1 8-K