<PAGE>
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                                 UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 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, 1998
 
                                       or
 
/ / TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE
    ACT OF 1934
 
    FOR THE TRANSITION PERIOD FROM:              COMMISSION FILE NUMBER:
                                                         33-46620
 
                            ------------------------
 
                       FORTIS BENEFITS INSURANCE COMPANY
             (Exact name of registrant as specified in its charter)
 
               MINNESOTA                                81-0170040
     State or other jurisdiction of                   (IRS Employer
     incorporation or organization                 Identification No.)
 
500 BIELENBERG DRIVE, WOODBURY, MN 55125
(Address of principal executive offices)
 
                 Registrant's telephone number: (612) 738-4000
 
                            ------------------------
 
        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 Sections 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
 
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<PAGE>
                      DOCUMENTS INCORPORATED BY REFERENCE
 
    Portions of the Form S-1 Amended Registration Statement to be filed by the
Registrant are incorporated by reference into Parts I, II, III.
 
                                     PART I
 
ITEM 1.  BUSINESS
 
    "Fortis Benefits/Fortis Financial Group Member" on page 10, and Further
Information About Fortis Benefits" on pages 23 through 24 of the prospectus
attached hereto as Exhibit No. 99 are incorporated herein.
 
    Fortis Benefits seeks to compete primarily on the basis of customer service,
product design, and, in the case of variable products, the investment results
achieved. Many other insurance companies compete with Fortis Benefits in each of
its markets, including on the basis of price. Many of these companies, which
include some of the largest and best known insurance companies, have
considerably greater resources than Fortis Benefits.
 
    The Company is subject to regulation and supervision by the insurance
departments of the states in which it is licensed to do business. This
regulation covers a variety of areas, including benefit reserve requirements,
adequacy of insurance company capital and surplus, various operational
standards, and accounting and financial reporting procedures. Fortis Benefits'
operations and accounts are subject to periodic examination by insurance
regulatory authorities.
 
    Under insurance guaranty fund laws in most states, insurers doing business
therein can be assessed up to prescribed limits for insurance contract losses,
if covered, incurred by insolvent companies. The amount of any future
assessments of Fortis Benefits under these laws cannot be reasonably estimated.
Most of these laws do provide, however, that an assessment may be excused or
deferred if it would threaten an insurer's own financial strength.
 
    Although the federal government generally does not directly regulate the
business of insurance, federal initiatives often have an impact on the business
in a variety of ways. Federal measures that may adversely affect the insurance
business include health care reform, employee benefit regulation, controls on
medicare costs and medical entitlement programs, tax law changes affecting the
taxation of insurance companies or of insurance products, changes in the
relative desirability of various personal investment vehicles, and removal of
impediments on the entry of banking institutions into the business of insurance.
 
    Pursuant to state insurance laws and regulations, Fortis Benefits is
obligated to carry on its books, as liabilities, reserves to meet its
obligations under outstanding insurance contracts. These reserves are based on
assumptions about, among other things, future claims experience and investment
returns. Neither the reserve requirements nor the other aspects of state
insurance regulation provide absolute protection to holders of insurance
contracts, if Fortis Benefits were to incur claims or expenses at rates
significantly higher than expected or significant unexpected losses on its
investments.
 
ITEM 2.  PROPERTIES
 
    Fortis Benefits has approximately 2,200 employees and considers its employee
relations to be excellent; Fortis Benefits owns its Home Office building,
consisting of 295,000 square feet in Woodbury, Minnesota. It also has
administrative offices in Kansas City, Missouri. Fortis Benefits leases a
portion of that building consisting of 297,000 square feet. In addition Fortis
Benefits has several regional claims and sales offices throughout the United
States. Fortis Benefits occupies approximately 100% of its home office and 70%
of its administration building, which it expects will be adequate for its
purposes for the foreseeable future.
 
ITEM 3.  LEGAL PROCEEDINGS
 
    The Company is a defendant in various lawsuits, none of which, in the
opinion of the Company counsel, will result in a material liability.
 
ITEM 4.  SUBMISSION OF MATTERS TO A VOTE OF SECURITY HOLDERS
 
    Not Applicable.
<PAGE>
                                    PART II
 
ITEM 5.  MARKET FOR REGISTRANT'S COMMON EQUITY AND RELATED STOCKHOLDER MATTERS
 
    Not applicable.
 
ITEM 6.  SELECTED FINANCIAL DATA
 
    "Selected Financial Data" from page 23 of the prospectus attached hereto as
Exhibit No. 99, is incorporated herein by reference.
 
ITEM 7.  MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
         OF OPERATIONS
 
    "Management's Discussion and Analysis of Financial Condition and Results of
Operations" on pages 23 through 25 of the prospectus attached hereto as Exhibit
No. 99 is incorporated herein by reference.
 
ITEM 7A.  QUANTITATIVE AND QUALITATIVE DISCLOSURES ABOUT MARKET RISK
 
    The matters set forth under the caption "Market Risk and Risk Management" in
Management's Discussion and Analysis of Results of Operations (Item 7 of this
report) are incorporated herein by reference.
 
ITEM 8.  FINANCIAL STATEMENTS AND SUPPLEMENTARY DATA
 
    "FORTIS BENEFITS Financial Statements" contained in the prospectus attached
hereto as Exhibit No. 99 is incorporated by reference.
 
ITEM 9.  CHANGES IN AND DISAGREEMENTS WITH ACCOUNTANTS ON ACCOUNTING AND
         FINANCIAL DISCLOSURE
 
    None.
<PAGE>
                                    PART III
 
ITEM 10.  DIRECTORS AND EXECUTIVE OFFICERS OF THE REGISTRANT
 
    Set forth is information concerning the Company's directors and executive
officers, to the extent responsible for its variable annuity operations,
together with their business experience and principal occupations for the past
five years:
 
<TABLE>
<S>                         <C>
OFFICER-DIRECTORS
Dean C. Kopperud, 46        Senior Vice President--Marketing and Sales; also
Director since 1995         officer of affiliated companies.
Robert Brian Pollock, 44    President and Chief Executive Officer; before then
Director Since 1988         Senior Vice President--Life and Disability.
Michael John Peninger, 44   Executive Vice President--Operations and Finance and
Director since 1998         Chief Financial Officer
OTHER DIRECTORS
Allen Royal Freedman, 58    Chairman and Chief Executive Officer of Fortis, Inc.
Chairman of the Board
since 1995
J. Kerry Clayton, 53        Executive Vice President of Fortis, Inc.
Director Since 1997
Arie Aristide Fakkert, 55   Assistant General Manager of Fortis International
Director Since 1987         N.V.
Alan W. Feagin              President and CEO of United Family Life Insurance
Director since 1998         Company
EXECUTIVE OFFICERS
Rhonda Schwartz, 40         Senior Vice President and General Counsel--Life and
                            Investment Products; before then secretary and
                            General Counsel of Fortis Inc.
Jon H. Nicholson, 49        Senior Vice President--Custom Solutions Group.
Peggy L. Ettestad, 41       Senior Vice President--Life Operations; before that
                            Vice President of General Electric Company.
Melinda S. Urion, 45        Senior Vice President--Chief Financial Officer since
                            1997; before then Senior Vice President--Finance &
                            CFO of American Express Financial Corporation.
Dickson W. Lewis, 50        Senior Vice President--Distribution and Marketing
                            since 1997; before then President of
                            Hedstrom/Blessing Marketing.
</TABLE>
 
    Fortis Benefits' officers serve at the pleasure of the board of directors,
and members of the board serve without compensation (except for expenses of
attending board meetings), until their successors are duly elected and
qualified.
 
    Mr. Freedman is a director of Systems and Computer Technology Corporation
and Genesis Health Ventures. Mr. Freedman is also a director of the following
registered investment companies: Fortis Equity Portfolios, Inc.; Fortis Growth
Fund, Inc.; Fortis Fiduciary Fund, Inc., Fortis Income Portfolios, Inc.; Fortis
Securities, Inc.; Fortis Tax-Free Portfolios, Inc.; Fortis Money Portfolios,
Inc.; Fortis Advantage Portfolios, Inc.; Fortis World Wide Portfolios, Inc.;
Fortis Series Fund, Inc.; Special Portfolios, Inc.
 
ITEM 11.  EXECUTIVE COMPENSATION
 
    Set forth below is certain information concerning the compensation of the
executive officers of Fortis Benefits.
<PAGE>
                           SUMMARY COMPENSATION TABLE
 
<TABLE>
<CAPTION>
                                                                 ANNUAL COMPENSATION                  LONG-TERM COMPENSATION
                                                      -----------------------------------------  --------------------------------
                                                                               OTHER ANNUAL                         ALL OTHER
       NAME AND PRINCIPAL POSITION           YEAR      SALARY      BONUS       COMPENSATION      LTIP PAYOUTS    COMPENSATION(1)
-----------------------------------------  ---------  ---------  ---------  -------------------  -------------  -----------------
<S>                                        <C>        <C>        <C>        <C>                  <C>            <C>
Robert B. Pollock                               1998  $ 300,000  $ 126,225       $       0         $       0        $  16,439
 President and Chief Executive Officer          1997    275,000     56,817               0                 0           15,762
                                                1996    215,000     69,660               0                 0           15,318
---------------------------------------------------------------------------------------------------------------------------------
Francis J. Guthrie                              1998    250,000     91,650               0                 0           19,699
 Executive Vice President                       1997    235,000    140,388               0                 0           15,762
                                                1996     88,125          0               0                 0           12,279
---------------------------------------------------------------------------------------------------------------------------------
Clifford S. Korte                               1998    150,000     60,840                            41,796           13,050
 Sr. Vice President--                           1997    144,000     26,133                                             10,208
 Underwriting                                   1996    130,000     39,975                                             10,198
---------------------------------------------------------------------------------------------------------------------------------
William D. Greiter                              1998    195,000     50,490               0                 0           15,129
 Senior Vice President--                        1997    187,000     48,195               0                 0           14,112
 Provider Markets                               1996    178,500     48,195               0                 0           12,829
---------------------------------------------------------------------------------------------------------------------------------
Michael John Peninger                           1998    230,000     78,000               0                 0           16,439
 Executive Vice President and                   1997    200,000     31,194               0                 0           13,872
 Chief Financial Officer                        1996    165,000     51,975               0                 0           13,018
</TABLE>
 
------------------------------
(1)  This column includes contributions made by Fortis Benefits for the year for
     the benefit for the named individual to a defined contribution retirement
     plan.
 
                     LONG-TERM INCENTIVE PLAN AWARDS TABLE
            (LONG-TERM INCENTIVE PLAN(1) AWARDS IN LAST FISCAL YEAR)
 
<TABLE>
<CAPTION>
                                                         PERFORMANCE OR       ESTIMATED FUTURE PAYOUTS UNDER
                                     NUMBER OF SHARES,    OTHER PERIOD          NON-STOCK PRICE BASED PLANS
                                      UNITS OR OTHER    UNTIL MATURATION  ---------------------------------------
NAME                                      RIGHTS           OR PAYOUT      THRESHOLD     TARGET        MAXIMUM
-----------------------------------  -----------------  ----------------  ---------  ------------  --------------
<S>                                  <C>                <C>               <C>        <C>           <C>
Robert B. Pollock..................        525 Units          3 years      0 Units      525 Units     1,575 Units
Francis J. Guthrie.................        292 Units          3 years      0 Units      292 Units       876 Units
Clifford S. Korte..................         88 Units          3 years      0 Units       88 Units       264 Units
William D. Greiter.................        152 Units          3 years      0 Units      152 Units       456 Units
Michael John Peninger..............        268 Units          3 years      0 Units      268 Units       804 Units
</TABLE>
 
------------------------
(1) Units shown in this table represent performance units granted pursuant to an
    Executive Incentive Compensation Plan in which officers and managers of
    Fortis Benefits participate. Awards are made pursuant to this plan based on
    the employee's position with Fortis Benefits and salary level and the extent
    to which the employee and Fortis Benefits meet certain performance
    objectives over 1- and 3-year periods. Employees may elect to defer awards
    payable to them under this plan.
 
    As additional compensation to its employees and executive officers, Fortis
Benefits has an Employees' Uniform Retirement Plan and an Executive Retirement
Plan which generally provide an annual annuity benefit upon retirement at age 65
(or a reduced benefit upon early retirement) equal to: .9% of the employee's
Average Annual compensation up to the employee's social security covered
compensation, plus 1.3% of compensation above the social security covered
compensation, up to $255,300, as adjusted by an index, multiplied by the
employee's years of credited services.
 
    In addition, Fortis Benefits provides an unfunded Supplemental Executive
Retirement Plan for certain executives of Fortis Benefits. Mr. Pollock is the
only named executive currently covered by the Plan. Under the Supplemental
Executive Retirement Plan, the annual benefit is calculated by subtracting the
benefit payable under the Employees' Uniform Retirement Plan and the estimated
Social Security benefit from the "Target Benefit." The "Target Benefit" is equal
to 50% of Final Average Salary (average salary over the final 36 consecutive
months of employment) reduced for less than 20 years of service at retirement.
Upon retirement prior to age 65 and after attaining age 55 with 10 years of
service, special early retirement rules apply. The salary used to calculate the
Final Average Salary
<PAGE>
consists of regular compensation and the annual target incentive bonus of the
participant. The estimated annual benefit of Mr. Pollock, based on current
compensation levels, under this plan is $187,572.
 
    The following table illustrates the COMBINED estimated life annuity benefit
payable from the Employees' Uniform Retirement Plan and Executive Retirement
Plan to employees with the specified Final Average Salary and years of service
upon retirement.
 
                              PENSION PLAN TABLE*
 
<TABLE>
<CAPTION>
                                                      YEARS OF SERVICE
                             ------------------------------------------------------------------
FINAL AVERAGE SALARY            10         15         20         25         30          35
---------------------------  ---------  ---------  ---------  ---------  ---------  -----------
<S>                          <C>        <C>        <C>        <C>        <C>        <C>
$125,000...................    $15,005  $  22,507  $  30,010  $  37,512  $  45,015  $    52,517
 150,000...................     18,255     27,382     36,510     45,637     54,765       63,892
 175,000...................     21,505     32,257     43,010     53,762     64,515       75,267
 200,000...................     24,755     37,132     49,510     61,887     74,265       86,642
 225,000...................     28,005     42,007     56,010     70,012     84,015       98,017
 250,000...................     30,990     46,485     61,980     77,475     92,970      108,465
 275,000+..................     31,908     47,862     63,816     79,770     95,724      111,678
</TABLE>
 
------------------------
* The table excludes social security benefits. In general, for the purposes of
  these plans, compensation includes salary and bonuses. The credited years of
  service with Fortis Benefits for these individuals named in the Summary
  Compensation Table above are as follows: 19, 5, 28, 15 and 14, respectively.
 
ITEM 12.  SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT
 
<TABLE>
<CAPTION>
                                                                    PERCENTAGE
                                                    NUMBER OF     OF OUTSTANDING
     NAME AND ADDRESS OF BENEFICIAL OWNER(1)         SHARES       VOTING SHARES
-------------------------------------------------  -----------  ------------------
<S>                                                <C>          <C>
Fortis, Inc.                                         1,000,000         100%
 One World Trade Center
 Suite 5001
 New York, NY 10048
</TABLE>
 
------------------------
 
(1) All of Fortis Benefits' outstanding shares are owned by Fortis Insurance
    Company, 515 West Wells, Milwaukee, WI, 53201, which is itself wholly owned
    by Fortis, Inc., One World Trade Center, Suite 5001, New York, NY 10048.
    Fortis, Inc. in turn is wholly owned by Fortis International, Inc., which is
    wholly owned by AMEV/VSB 1990 N.B. both of which share the same address with
    Fortisl(NL) N.V., Archimiedeslaan 10, 3584 BA, Utrecht, The Netherlands.
    AMEV/VSB 1990 N.V. is 50% owned by Fortis(NL) N.V. and 50% owned, through
    certain subsidiaries, by Fortis(B), Boulevard Emile Jacqmain 53, 1000
    Brussels, Belgium.
 
ITEM 13.  CERTAIN RELATIONSHIPS AND RELATED TRANSACTIONS
 
    None.
<PAGE>
                                    PART IV
 
ITEM 14.  EXHIBITS, FINANCIAL STATEMENT SCHEDULES AND REPORTS ON FORM 8-K
 
(a)(1)The following financial statements of Fortis Benefits Insurance Company
      are included in Item 8:
 
      Report of Independent Auditors
 
      Balance Sheets at December 31, 1998 and 1997
 
      Statements of Income for the years ended December 31, 1998, 1997 and 1996
 
      Statements of Changes in Shareholder's Equity for the years ended December
      31, 1998, 1997 and 1996
 
      Statements of Cash Flows for the years ended December 31, 1998, 1997 and
      1996.
 
      Notes to Financial Statements
 
(a)(2)The information required by the following financial statement schedules of
      Fortis Benefits Insurance Company are included in Item 8:
 
      I.  Summary of Investments--Other than investments in Related
      Parties--Contained in the Notes to Financial Statements.
 
      II. Condensed Financial Information of Registrant--Contained in
      non-financial statement part of prospectus.
 
      III. Supplementary Insurance Information--Contained in Financial
      Statements and Notes to Financial Statements.
 
      IV. Reinsurance--Contained in the Notes to Financial Statements.
 
      V.  Valuation and Qualifying Accounts--Contained in Financial Statements
      and Notes to Financial Statements.
 
    All other schedules to the financial statements required by Article 7 of the
Regulation S-X are not required under the related instructions or are
inapplicable and therefore have been omitted.
 
  (3) Listing of Exhibits
 
      3.(a) Articles of Incorporation of Fortis Benefits Insurance Company
      (incorporated by reference from Form S-6 Registration Statement of Fortis
      Benefits and its Variable Account C filed on March 17, 1986, File No.
      33-03919);
 
        (b) By-laws of Fortis Benefits Insurance Company (incorporated by
      reference from Form S-6 Registration Statement of Fortis Benefits and its
      Variable Account C filed on March 17, 1986, File No. 33-03919);
 
        (c) Amendments to Articles of Incorporation and By-laws dated November
      21, 1991 (incorporated by reference from Post-Effective Amendment No. 1 to
      the Form N-4 Registration Statement of Fortis Benefits and its Variable
      Account D filed on March 2, 1992, File No. 33-37577).
 
      4.(a) Form of Combination Fixed and Variable Group Annuity Contract
      (incorporated by reference from Post-Effective Amendment No. 1 to the Form
      N-4 Registration Statement of Fortis Benefits and its Variable Account D
      filed on March 2, 1992, File No. 33-37577);
 
        (b) Form of Certificate to be used in connection with Contract filed as
      Exhibit 4(a) (incorporated by reference from the Post-Effective Amendment
      No. 1 to the Form N-4 Registration Statement of Fortis Benefits and its
      Variable Account D filed on March 2, 1992, File No. 33-37577);
<PAGE>
        (c) Form of Application to be used in connection with Certificate filed
      as Exhibit 4(b) (incorporated by reference from Post-Effective Amendment
      No. 1 to the Form N-4 Registration Statement of Fortis Benefits and its
      Variable Account D filed on March 2, 1992, File No. 33-37577);
 
        (d) Form of IRA Endorsement (incorporated by reference from
      Pre-Effective Amendment No. 1 to Form N-4 Registration Statement of Fortis
      Benefits and its Variable Account D filed on March 28, 1991, File No.
      33-37577);
 
        (e) Form of Section 403(b) Annuity Endorsement (incorporated by
      reference from Post-Effective Amendment No. 3 to the Form N-4 Registration
      Statement of Fortis Benefits and its Variable Account D filed on March 1,
      1990, File No. 33-19421);
 
        (f) Annuity Contract Exchange Form (incorporated by reference from
      Pre-Effective Amendment No. 1 to the Form N-4 Registration Statement of
      Fortis Benefits and its Variable Account D filed on April 19, 1988, File
      No. 33-19421).
 
      10. Fortis, Inc. Executive Incentive Compensation Plan (incorporated by
      reference from Amendment No. 1 to Form S-1 Registration Statement of
      Fortis Benefits filed on March 28, 1991, File No. 33-37576).
 
      24. Power of Attorney for Messrs. Freedman, Gaddy, Mackin, Meler, Mahoney,
      Clancy, Keller, Greiter and Clayton (incorporated by reference from
      Exhibit 11 to Form S-6 registration statement of Fortis Benefits, File No.
      33-73138 filed on December 17, 1993).
 
      99. Form of prospectus to be filed as part of Form S-1 Amended
      Registration Statement of Fortis Benefits.
 
(b)   Reports on Form 8-K filed in the fourth quarter of 1998
 
      None
 
(c)   Exhibits
 
      Included in 14 (a)(3) above
 
(d)   Financial Statements Schedules
 
      Included in 14 (a)(2) above
<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.
 
                                            FORTIS BENEFITS INSURANCE COMPANY
                                                        Registrant
 
March 27, 1999                  By              /s/ ROBERT B. POLLOCK
                                      -----------------------------------------
                                                 Robert B. Pollock,
                                                    PRESIDENT AND
                                               CHIEF EXECUTIVE OFFICER
 
March 27, 1999                  By             /s/ MICHAEL J. PENINGER
                                      -----------------------------------------
                                                Michael J. Peninger,
                                              SENIOR VICE PRESIDENT AND
                                               CHIEF FINANCIAL OFFICER
 
    Pursuant to the requirements of the Securities Exchange Act of 1934, this
report has been duly signed below by the following persons on behalf of the
registrant and in the capacities and on the dates indicated. The following
persons represent a majority of the Board of Directors of Fortis Benefits
Insurance Company:
 
By*                                         Chairman of the       March 27, 1999
     -------------------------------------   Board
             Allen Royal Freedman
 
By           /s/ ROBERT B. POLLOCK          President and Chief   March 27, 1999
     -------------------------------------   Executive Officer
               Robert B. Pollock
 
By           /s/ DEAN C. KOPPERUD           Director              March 27, 1999
     -------------------------------------
               Dean C. Kopperud
 
By*                                         Director              March 27, 1999
     -------------------------------------
               J. Kenny Clayton
 
By          /s/ MICHAEL J. PENINGER         Director              March 27, 1999
     -------------------------------------
              Michael J. Peninger
 
*By          /s/ ROBERT B. POLLOCK
     -------------------------------------
              Robert B. Pollock,
               ATTORNEY-IN-FACT