UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549
                                    Form 10-K

[X]  Annual report  pursuant to Section 13 or 15(d) of the  Securities  Exchange
     Act of 1934 for the fiscal year ended December 31, 2003

[ ]  Transition  report  pursuant  to Section  13 or 15(d) of the  Securities
     Exchange Act of 1934

     for the transition period from       to


              Commission File Number 333-67140-07

                   WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
             (Exact name of Registrant as specified in its charter)

             Delaware                            94-2528990
 (State or other jurisdiction of    (I.R.S. Employer Identification No.)
          incorporation

                           1201 THIRD AVENUE, WMT 1706
                            SEATTLE, WASHINGTON 98101
                                 (206) 377-8555
                   (Address, including zip code, and telephone
                         number, including area code, of
                        registrant's principal executive
                                    offices)

        WASHINGTON MUTUAL MORTGAGE SECURITIES CORP. MORTGAGE PASS-THROUGH
                         CERTIFICATES, SERIES 2001-MS12
             (Title of each class of securities covered bythis Form)

Securities registered pursuant to Section 12(b) of the Act: None
Securities registered pursuant to Section 12(g) of the Act: None

     Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to such
filing requirements for the past 90 days. Yes [X] No [ ] (as such reporting
obligations are modified as set forth in J.P. Morgan Mortgage Pass-Through
Corporation, SEC No-Action Letter, March 31, 1989, in reliance upon which the
registrant is filing this modified report).

<PAGE>

     Indicate by check mark if disclosure of delinquent filers pursuant to Item
405 of Regulation S-K is not contained herein, and will not be contained, to the
best of registrant's knowledge, in definitive proxy or information statements
incorporated by reference in Part III of this Form 10-K or any amendment to this
Form 10-K. [ ] Not applicable.

     State the aggregate market value of the voting and non-voting common equity
held by non-affiliates of the registrant: None.

     Indicate the number of shares outstanding of each of the registrant's
classes of common stock, as of the latest practicable date: Not applicable.

                       DOCUMENTS INCORPORATED BY REFERENCE

     The registrant's Reports on Form 8-K concerning the registrant's WASHINGTON
MUTUAL MORTGAGE  SECURITIES CORP.  MORTGAGE  PASS-THROUGH  CERTIFICATES,  SERIES
2001-MS12,  issued  pursuant to the Pooling and Servicing  Agreement dated as of
October  1,  2001  (the  "Pooling  Agreement")  and  registered  pursuant  to  a
registration   statement  on  Form  S-3  (File  No.  333-67140)  (such  mortgage
pass-through certificates, the "Certificates"), are incorporated by reference as
set forth in Part IV, Item 15.  Capitalized  terms used herein but not otherwise
defined shall have the meanings assigned to them in the Pooling Agreement.

                                     Part I

Item 1. Business.

     Not applicable.

Item 2. Properties.

     Information regarding the mortgaged properties related to the Certificates
is furnished by reference to the Annual Statement of Compliance attached hereto
as Exhibit 99.1.

Item 3. Legal Proceedings.

     There are no material pending legal proceedings with respect to the assets
of the Trust involving the registrant or, to the best knowledge of the
registrant, the Trustee, any Custodian, any Servicer or the assets of the Trust,
other than ordinary routine litigation incidental to the Trustee's, any
Custodian's, any Servicer's or the registrant's duties under the Pooling
Agreement.

Item 4. Submission of Matters to a Vote of Security Holders.

In May, June and July 2003, the registrant solicited the consent of
Certificateholders to a proposed amendment to the Pooling Agreement. The
amendment proposed, among other things, to (i) reduce the amount of the
servicing fee with respect to some mortgage loans serviced by Washington Mutual
Bank, FA to a level sufficient to maintain the ratings assigned to the
Certificates; (ii) identify the amount by which these servicing fees were
reduced as the "Excess Yield Amount" for each affected mortgage loan; (iii)
identify Washington Mutual Bank, FA as the initial holder of the interest in
such Excess Yield Amounts; (iv) provide for the payment of the Excess Yield
Amounts to such holder; and (v) provide for the registration of transfer of such
interest in the Excess Yield Amounts to one or more subsequent holders.

Holders of Certificates evidencing an aggregate Percentage Interest (as defined
in the Pooling Agreement) of approximately 41.0% consented to the proposed
amendment. Holders of Certificates evidencing an aggregate Percentage Interest
of approximately 59.0% did not respond to the solicitation or otherwise withheld
their consent.



<PAGE>


                                     Part II

     Item 5. Market for Registrant's Common Equity and Related Stockholder
Matters.

     (a) To the knowledge of the registrant, there exists no established public
trading market for the Certificates.

     (b) The Certificates are represented by one or more certificates registered
in the name of Cede & Co., as nominee of the Depository Trust Company ("DTC")
and are held in book-entry form only. The registrant considers Cede & Co. to be
the sole holder of record of the Certificates.

     (c) Not applicable.

Item 6. Selected Financial Data.

         Not applicable.

Item 7. Management's Discussion and Analysis of Financial Condition and Results
of Operations.

     Not applicable.

Item 7A. Quantitative and Qualitative Disclosures About Market Risk.

     Not applicable.

Item 8. Financial Statements and Supplementary Data.

     Not applicable.

Item 9. Changes in and Disagreements with Accountants on Accounting and
Financial Disclosure.

         None.

                                    Part III


Item 10. Directors and Executive Officers of the Registrant.

     Not applicable.

Item 11. Executive Compensation.

     Not applicable.

Item 12. Security Ownership of Certain Beneficial Owners and Management.

     Not applicable.

Item 13. Certain Relationships and Related Transactions.

     Not applicable.

Item 14. Principal Accountant Fees and Services.

     Not applicable.

<PAGE>

                                     Part IV

Item 15. Exhibits, Financial Statement Schedules, and Reports on Form 8-K.

     (a) Documents filed as part of this report.

     1. Not applicable.

     2. Not applicable.

     3. The required exhibits are as follows:

<TABLE>
<CAPTION>
  Exhibit No.       Description
<S>                 <C>
31                  Rule 13a-14(d) Certification

99.1                Officer's Certificate regarding Annual Statement of Compliance of
                    Washington Mutual Mortgage Securities Corp., dated March 15, 2004

99.2                Independent Accountants' Report on Management's Assertion Concerning
                    Compliance with Washington Mutual Mortgage Securities Corp.'s Minimum
                    Servicing Standards as of and for the year ended December 31, 2003
                    (incorporated by reference to the designated exhibit included in the
                    registrant's Current Report on Form 8-K, filed on March 24, 2004,
                    (SEC File No. 333-67140))

</TABLE>


     (b) Reports on Form 8-K.

     The registrant filed Current Reports on Form 8-K that described
distributions made to Certificateholders on the Distribution Date occurring in
the month preceding the month in which such Current Report on Form 8-K was filed
and also provided certain information regarding delinquent Mortgage Loans and
credit enhancements as of such Distribution Date. Such Current Reports on Form
8-K are incorporated by reference herein, as follows:

WASHINGTON MUTUAL MORTGAGE SECURITIES CORP. MORTGAGE PASS-THROUGH  CERTIFICATES,
SERIES 2001-MS12


EVENT REPORTED
Monthly Distribution Report
sent to holders of the Certificates      Date Distribution Report Filed
for payments made in:                    on Form 8-K:
------------------------------------     ------------------------------

May 2003                                 June 10, 2003
June 2003                                July 8, 2003
July 2003                                August 8, 2003
August 2003                              September 5, 2003
September 2003                           October 9, 2003
October 2003                             November 4, 2003
November 2003                            December 4, 2003


<PAGE>



                                   SIGNATURES

     Pursuant to the requirements of Section 13 or 15(d) of the Securities
Exchange Act of 1934, the registrant has duly caused this report to be signed on
its behalf by the undersigned, thereunto duly authorized.

Dated:  March 24, 2004         WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.


                          By:  /s/ Michael L. Parker
                               Michael L. Parker,
                               President and Director