<PAGE>

                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549

                               FORM 10-K/A No. 1

[X]  Annual Report Pursuant to Section 13 or 15(d) of the Securities Exchange
     Act of 1934 For the fiscal year ended December 31, 1999

                                      OR

[_]  Transition Report Pursuant to Section 13 or 15(d) of the Securities
     Exchange Act of 1934.

                       Commission file number 001-12617

                            Trigon Healthcare, Inc.
            (Exact name of registrant as specified in its charter)

                Virginia                                   54-1773225
      (State or other jurisdiction of                   (I.R.S. Employer
       incorporation or organization)                  Identification No.)


                  2015 Staples Mill Road, Richmond, VA 23230
                   (Address of principal executive offices)

       Registrant's telephone number, including area code (804) 354-7000

Securities registered pursuant to Section 12(b)
                  of the Act:
   Class A Common Stock, $.01 Par Value             New York Stock Exchange
         (Title of Class)                             (Name of Exchange)

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.  [X] Yes [ ] No

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.  [X]

The aggregate market value of the voting stock held by non-affiliates of the
registrant as of April 24, 2000 was approximately $1,424,133,000 (based on the
last reported sales price of $37 13/16 per share on April 24, 2000, on the New
York Stock Exchange).

As of April 24, 2000, 37,663,024 shares of the registrant's Class A Common
Stock, par value $.01 per share, were issued and outstanding.

                     DOCUMENTS INCORPORATED BY REFERENCE:
Certain portions of Trigon Healthcare Inc.'s Annual Report to Shareholders for
the year ended December 31, 1999 into Parts II and IV of this Form 10-K.

Certain portions of Trigon Healthcare Inc.'s definitive Proxy Statement dated
March 24, 2000 for the Annual Meeting of Shareholders into Part III of this Form
10-K.
<PAGE>

The exhibits to this Form 10-K/A No. 1 are amended to include the 1999 annual
reports for the Trigon Healthcare, Inc. Employee Stock Purchase Plan and the
Trigon Insurance Company 401(k) Restoration Plan and Consents of Independent
Auditors. There are no other differences.

PART I

ITEM 1.  BUSINESS
Omitted

ITEM 2.  PROPERTIES
Omitted

ITEM 3.  LEGAL PROCEEDINGS
Omitted

ITEM 4.  SUBMISSION OF MATTERS TO A VOTE OF SECURITY HOLDERS
Not applicable.

PART II

ITEM 5.  MARKET FOR REGISTRANT'S COMMON EQUITY AND RELATED STOCKHOLDER MATTERS
Omitted

ITEM 6.  SELECTED FINANCIAL DATA
Omitted

ITEM 7.  MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
OF OPERATIONS
Omitted

ITEM 7A.  QUANTITATIVE AND QUALITATIVE DISCLOSURES ABOUT MARKET RISK
Omitted

ITEM 8.  FINANCIAL STATEMENTS AND SUPPLEMENTARY DATA
Omitted

ITEM 9.  CHANGES IN AND DISAGREEMENTS WITH ACCOUNTANTS ON ACCOUNTING AND
FINANCIAL DISCLOSURE
Not applicable.

PART III

Item 10.  DIRECTORS AND EXECUTIVE OFFICERS OF THE REGISTRANT
Omitted
<PAGE>

ITEM 11.  EXECUTIVE COMPENSATION
Omitted

ITEM 12.  SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT
Omitted

ITEM 13.  CERTAIN RELATIONSHIPS AND RELATED TRANSACTIONS
Omitted

PART IV

ITEM 14. EXHIBITS, FINANCIAL STATEMENT SCHEDULES, AND REPORTS ON FORM 8-K
(a) The following documents are filed as part of this report.
     1.  Consolidated Financial Statements from Trigon Healthcare Inc.'s 1999
         Annual Report to Shareholders are incorporated herein by reference in
         Item 8:
         Omitted

     2.  Financial statement schedules
         Omitted

     3.  Exhibits.  The following is a list of exhibits to this Form 10-K.

Exhibit
Number            Description
------            -----------

      2    -- Amended and Restated Plan of Demutualization.  (1)
    3.1    -- Amended and Restated Articles of Incorporation of Trigon
              Healthcare, Inc.  (1)
    3.2    -- Amended and Restated Bylaws of Trigon Healthcare, Inc. dated
              April 28, 1999.  (9)
    3.3    -- Articles of Amendment to Amended and Restated Articles of
              Incorporation setting forth the designation, preferences and
              rights of Series A Junior Participating Preferred Stock of Trigon
              Healthcare, Inc. dated July 16, 1997.  (4)
      4    -- Form of Stock Certificate (other Instruments Defining the Rights
              of Security-Holders).  (1)
    4.1    -- Rights Agreement dated as of July 16, 1997 between Trigon
              Healthcare, Inc. and First Chicago Trust Company of New York, as
              Rights Agent.  (4)
    4.2    -- Form of Rights Certificate.  (4)
   10.1    -- License Agreement by and between the Blue Cross and Blue Shield
              Association and the Company.  (2)
                  (a) Blue Cross license
                  (b) Blue Shield license
   10.2    -- Limited Fixed Return Plan for Certain Officers and Directors of
              Trigon Insurance Company.  (1) *
   10.4    -- Non-Contributory Retirement Program for Certain Employees of
              Trigon Insurance Company.  (1) *
<PAGE>

   10.5    -- Amended and Restated Supplemental Executive Retirement Program for
              Certain Employees of Trigon Insurance Company dated as of October
              1, 1998.  (8)*
   10.6    -- Salary Deferral Plan for Norwood H. Davis, Jr.  (1) *
   10.7    -- Amended and Restated Employment Agreement dated September 16, 1998
              by and between Trigon Insurance Company and Norwood H. Davis,
              Jr.  (7) *
   10.9    -- Amended and Restated Employees' Thrift Plan of Trigon Insurance
              Company dated as of October 1, 1998.  (8)*
  10.10    -- Amended and Restated Trigon Insurance Company 401(k) Restoration
              Plan dated as of October 1, 1998.  (8)*
  10.12    -- Form of Employment Agreement dated as of December 12, 1990 by and
              between Trigon Insurance Company and John C. Berry
               and certain other executive officers.  (1) *
  10.14    -- Credit Agreement dated as of February 5, 1997 among Trigon
              Healthcare, Inc., the banks party thereto and Morgan Guaranty
              Trust Company of New York, as Agent.  (2)
  10.15    -- 1997 Stock Incentive Plan.  (6) *
  10.16    -- Employee Stock Purchase Plan.  (6) *
  10.17    -- Non-Employee Directors Stock Incentive Plan.  (6) *
  10.18    -- Amendment to the License Agreement by and between the Blue Cross
              and Blue Shield Association and the Company. (5)
  10.19    -- Amendment to the Non-Contributory Retirement Program for Certain
              Employees of Trigon Insurance Company. (3) *
  10.20    -- Form of Executive Continuity Agreement dated as of September 16,
              1998 between Trigon Insurance Company and Thomas G. Snead, Jr. and
              certain other executive officers.  (7) *
  10.21    -- Form of Executive Continuity Agreement dated as of September 16,
              1998 between Trigon Insurance Company and John C. Berry and
              certain other executive officers.  (7) *
  10.22    -- Amendment to the Non-Contributory Retirement Program for Certain
              Employees of Trigon Insurance Company (now to be known as) The
              Trigon Insurance Company Retirement Program dated as of October 1,
              1998.  (8) *
  10.23    -- Clarifying Amendment to the Non-Contributory Retirement Program
              for Certain Employees of Trigon Insurance Company (now to be known
              as) The Trigon Insurance Company Retirement Program dated as of
              October 1, 1998. (8) *
  10.24    -- Thomas G. Snead Employment Agreement dated May 19, 1999.  (10) *
  10.25    -- Amendment No. 1 to Executive Continuity Agreement Between Trigon
              Insurance Company and Thomas G. Snead, Jr. dated May 19, 1999.
              (10) *
  10.26    -- Clarifying Amendment No. 2 to the Non-Contributory Retirement
              Program for Certain Employees of Trigon Insurance Company (now to
              be known as) The Trigon Insurance Company Retirement Program.
              (11) *
  10.27    -- Clarifying Amendment No. 3 to the Non-Contributory Retirement
              Program for Certain Employees of Trigon Insurance Company (now to
              be known as) The Trigon Insurance Company Retirement Program.
              (11) *
  10.28    -- Clarifying Amendment No. 4 to The Trigon Insurance Company
              Retirement Program. (11) *
<PAGE>

 10.29    -- Amendment No. 1 dated December 14, 1999 to the Credit Agreement
              dated as of February 5, 1997 among Trigon Healthcare, Inc., the
              banks party thereto and Morgan Guaranty Trust Company of New York,
              as Agent. (11)
     11    -- Computation of per share earnings.  Refer to Note 15, "Net Income
              and Pro Forma Net Income Per Share," on page 49 of Trigon
              Healthcare Inc.'s 1999 Annual Report to Shareholders, which is
              incorporated herein by reference.
     13    -- Excerpts from the Company's Annual Report to Shareholders for the
              year ended December 31, 1999.  (11)
     21    -- Subsidiaries of the Registrant.  (11)
   23.1    -- Consent of Independent Auditors.  (11)
   23.2    -- Consent of Independent Auditors.
   23.3    -- Consent of Independent Auditors.
     27    -- Financial Data Schedule.  (11)
   99.1    -- Trigon Healthcare, Inc. Employee Stock Purchase Plan Audited
              Financial Statements as of December 31, 1999 and 1998 and for
              years ended December 31, 1999 and 1998 and the period May 1, 1997
              (inception) through December 31, 1997.
   99.2    -- Trigon Insurance Company 401(k) Restoration Plan Audited Financial
              Statements as of December 31, 1999 and 1998 and for the three
              years ended December 31, 1999, 1998 and 1997.

(1)  Incorporated by reference to exhibits filed with the Company's Registration
     Statement on Form S-1 (registration number 333-09773).
(2)  Incorporated by reference to exhibits filed with the Company's Form 10-K
     for the year ended December 31, 1996.
(3)  Incorporated by reference to exhibits filed with the Company's Form 10-K
     for the year ended December 31, 1997.
(4)  Incorporated by reference to exhibits filed with the Company's Form 8-A/A
     filed on July 16, 1997.
(5)  Incorporated by reference to exhibits filed with the Company's Form 10-Q
     for the period ended September 30, 1997.
(6)  Incorporated by reference to exhibits filed with the Company's Proxy
     Statement dated March 13, 1997.
(7)  Incorporated by reference to exhibits filed with the Company's Form 10-Q
     for the period ended September 30, 1998.
(8)  Incorporated by reference to exhibits filed with the Company's Form 10-K
     for the year ended December 31, 1998.
(9)  Incorporated by reference to exhibits filed with the Company's Form 10-Q
     for the period ended March 31, 1999.
(10) Incorporated by reference to exhibits filed with the Company's Form 10-Q
     for the period ended June 30, 1999.
(11) Incorporated by reference to exhibits filed with the Company's Form 10-K
     for the year ended December 31, 1999.

  *  Management contract or compensatory plan or arrangement required to be
     filed as an exhibit to this Form 10-K pursuant to Item 14(c) of this Form
     10-K.
<PAGE>

All other schedules for which provision is made in the applicable accounting
regulation of the Securities and Exchange Commission are not required under the
related instructions or are inapplicable, and therefore have been omitted.
<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 amendment to be signed on its
behalf by the undersigned, thereunto duly authorized.

                                             TRIGON HEALTHCARE, INC.
                                                       Registrant


                                           By:   /s/ THOMAS R. BYRD
                                                 ------------------
                                                   THOMAS R. BYRD

                                        Title:   SENIOR VICE PRESIDENT AND
                                                    CHIEF FINANCIAL OFFICER

Date:  April 26, 2000

Pursuant to the requirements of the Securities Exchange Act of 1934, this
amendment has been signed below by the following persons on behalf of the
registrant and in the capacities and on the dates indicated.


<TABLE>
<CAPTION>
        SIGNATURE                           TITLE                                  DATE
        ---------                           -----                                  ----
<S>                                    <C>                                     <C>


/s/ THOMAS G. SNEAD, JR.               Chairman of the Board, President        April 26, 2000
---------------------------------      and Chief Executive Officer
THOMAS G. SNEAD, JR                    (Principal Executive Officer)

/s/ THOMAS R. BYRD                     Senior Vice President and Chief         April 26, 2000
---------------------------------       Financial Officer  (Principal
THOMAS R. BYRD                          Financial and Accounting Officer)

/s/ LENOX D. BAKER, JR.                Director                                April 26, 2000
---------------------------------
LENOX D. BAKER, JR., M.D.
</TABLE>
<PAGE>

<TABLE>
<CAPTION>
SIGNATURE                              TITLE                                   DATE
---------                              -----                                   ----
<S>                                    <C>                                     <C>


/s/ ROBERT M. FREEMAN                  Director                                April 26, 2000
---------------------------------
ROBERT M. FREEMAN

/s/ WILLIAM R. HARVEY                  Director                                April 26, 2000
---------------------------------
WILLIAM R. HARVEY, Ph.D.

/s/ GARY A. JOBSON                     Director                                April 26, 2000
---------------------------------
GARY A. JOBSON

/s/ JOSEPH S. MALLORY                  Director                                April 26, 2000
---------------------------------
JOSEPH S. MALLORY

/s/ DONALD B. NOLAN                    Director                                April 26, 2000
---------------------------------
DONALD B. NOLAN, M.D.

/s/ WILLIAM N. POWELL                  Director                                April 26, 2000
---------------------------------
WILLIAM N. POWELL

/s/ J. CARSON QUARLES                  Director                                April 26, 2000
---------------------------------
J. CARSON QUARLES

/s/ R. GORDON SMITH                    Director                                April 26, 2000
---------------------------------
R. GORDON SMITH, ESQ.

/s/ HUBERT R. STALLARD                 Director                                April 26, 2000
---------------------------------
HUBERT R. STALLARD

/s/ JACKIE M. WARD                     Director                                April 26, 2000
---------------------------------
JACKIE M. WARD

/s/ STIRLING L. WILLIAMSON, JR.        Director                                April 26, 2000
---------------------------------
STIRLING L. WILLIAMSON, JR.
</TABLE>
<PAGE>

EXHIBIT INDEX

Exhibit
Number                     Description
------                     -----------

   23.2    -- Consent of Independent Auditors.

   23.3    -- Consent of Independent Auditors.

   99.1    -- Trigon Healthcare, Inc. Employee Stock Purchase Plan Audited
               Financial Statements as of December 31, 1999 and 1998 and for
               years ended December 31, 1999 and 1998 and the period May 1, 1997
               (inception) through December 31, 1997.

   99.2    -- Trigon Insurance Company 401(k) Restoration Plan Audited Financial
               Statements as of December 31, 1999 and 1998 and for the three
               years ended December 31, 1999, 1998 and 1997.