<PAGE>   1
                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549

                            ------------------------

                                    FORM 10-Q

     (MARK ONE)

        [X]     QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
                SECURITIES EXCHANGE ACT OF 1934

                FOR THE QUARTERLY PERIOD ENDED MARCH 31, 2000 OR

        [ ]     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 1-8789

                            ------------------------

                        AMERICAN SHARED HOSPITAL SERVICES
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)

          CALIFORNIA                                          94-2918118
(STATE OR OTHER JURISDICTION OF                             (IRS EMPLOYER
INCORPORATION OR ORGANIZATION)                            IDENTIFICATION NO.)

  TWO EMBARCADERO CENTER, SUITE 2370, SAN FRANCISCO, CALIFORNIA     94111-3823
            (ADDRESS OF PRINCIPAL EXECUTIVE OFFICES)                (ZIP CODE)

       REGISTRANT'S TELEPHONE NUMBER, INCLUDING AREA CODE: (415) 788-5300

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 OF MAY 3, 2000, THERE ARE OUTSTANDING 3,810,042 SHARES OF THE REGISTRANT'S
COMMON STOCK.


<PAGE>   2

                         PART I - FINANCIAL INFORMATION

ITEM 1.    FINANCIAL STATEMENTS

<TABLE>
<CAPTION>

                        AMERICAN SHARED HOSPITAL SERVICES
                      CONDENSED CONSOLIDATED BALANCE SHEETS


                                                         (unaudited)           (audited)
ASSETS                                                  Mar. 31, 2000        Dec. 31, 1999
----------------------------------------                --------------       -------------
<S>                                                     <C>                  <C>
Current assets:
     Cash and cash equivalents                          $   12,698,000       $   12,903,000
     Restricted cash                                            50,000               50,000
     Trade accounts receivable                               1,745,000              982,000
     Other receivables                                         123,000              244,000

     Prepaid expenses and other assets                         505,000              516,000
                                                        --------------       --------------


TOTAL CURRENT ASSETS                                        15,121,000           14,695,000


Property and equipment:

     Medical equipment and facilities                       23,565,000           23,560,000
     Office equipment                                          617,000              617,000
     Deposits and construction in progress                   3,389,000            3,276,000
                                                        --------------       --------------

                                                            27,571,000           27,453,000
     Accumulated depreciation and
          amortization                                      (5,915,000)          (5,397,000)
                                                        --------------       --------------
Net property & equipment                                    21,656,000           22,056,000

Other assets                                                   241,000              235,000
                                                        --------------       --------------

TOTAL ASSETS                                            $   37,018,000       $   36,986,000
                                                        ==============       ==============

 LIABILITIES AND SHAREHOLDERS' EQUITY

 Current liabilities:
      Accounts payable                                  $      397,000       $      101,000
      Interest payable                                         144,000              229,000
      Employee compensation and benefits                        81,000               87,000
      Other accrued liabilities                                515,000              597,000
      Current portion of accrued exit costs                     10,000               11,000
      Current portion of long-term debt                      2,793,000            2,545,000
                                                        --------------       --------------

 TOTAL CURRENT LIABILITIES                                   3,940,000            3,570,000

 Long-term debt, less current portion                       19,024,000           19,887,000
 Minority interest                                           1,037,000              890,000

 Shareholders' equity:
      Common stock, without par value:
           authorized shares - 10,000,000; issued
           & outstanding shares, 3,810,000 in 2000
           and 3,813,000 in 1999                            10,023,000           10,036,000

      Common stock options issued to officer                 2,414,000            2,414,000
      Additional paid-in capital                               817,000              817,000
      Accumulated deficit                                     (237,000)            (628,000)
                                                        --------------       --------------

 TOTAL SHAREHOLDERS' EQUITY                                 13,017,000           12,639,000
                                                        --------------       --------------

 TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY             $   37,018,000       $   36,986,000
                                                        ==============       ==============
</TABLE>




                             See accompanying notes

                                        2

<PAGE>   3

                        AMERICAN SHARED HOSPITAL SERVICES
                   CONDENSED CONSOLIDATED STATEMENTS OF INCOME
                                   (Unaudited)

<TABLE>
<CAPTION>

                                                 Three Months ended March 31,
                                                   2000               1999
                                               ------------       ------------
<S>                                            <C>                <C>
REVENUES:
     Medical services                          $  2,098,000       $  1,566,000

COSTS AND EXPENSES:
     Costs of operations:

          Medical services payroll                        0              4,000

          Maintenance and supplies                   30,000             28,000

          Depreciation and amortization             514,000            381,000

          Other                                     170,000             58,000
                                               ------------       ------------
                                                    714,000            471,000

     Selling and administrative                     548,000            426,000

     Interest                                       501,000            255,000
                                               ------------       ------------

Total costs and expenses                          1,763,000          1,152,000
                                               ------------       ------------

                                                    335,000            414,000


Interest and other income                           204,000            119,000

Minority interest                                  (148,000)           (90,000)
                                               ------------       ------------

Income before income taxes                          391,000            443,000

Income tax benefit                                        0             45,000
                                               ------------       ------------

Net income                                     $    391,000       $    488,000
                                               ============       ============

Net income per share:

     Earnings per common share - basic         $       0.10       $       0.11
                                               ============       ============

     Earnings per common share - assuming
          dilution                             $       0.07       $       0.08
                                               ============       ============
</TABLE>

                             See accompanying notes

                                       3
<PAGE>   4

                        AMERICAN SHARED HOSPITAL SERVICES
                 CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS
                                   (Unaudited)

<TABLE>
<CAPTION>

                                                                                Three Months ended March 31,
                                                                                   2000               1999
                                                                               ------------       ------------
<S>                                                                            <C>                <C>
OPERATING ACTIVITIES:
Net income                                                                     $    391,000       $    488,000

Adjustments to reconcile net cash provided by operating activities:

     Depreciation and amortization                                                  527,000            380,000

     Changes in operating assets and liabilities:

          Decrease (increase) in restricted cash                                          0            (73,000)

          (Increase) decrease in accounts receivable                               (642,000)            97,000

          (Increase) in prepaid expenses and other assets                            (5,000)           (15,000)

          Increase (decrease) in accounts payable and accrued liabilities           122,000         (1,745,000)
                                                                               ------------       ------------

Net cash from operating activities                                                  393,000           (868,000)

INVESTING ACTIVITIES:
     Purchase of property and equipment (net of financing)                         (118,000)           (67,000)

     Increase (decrease) in minority interest                                       148,000           (100,000)
                                                                               ------------       ------------

     Net cash from investing activities                                              30,000           (167,000)

FINANCING ACTIVITIES:
     Payment for repurchase of stock                                                (13,000)          (792,000)

     Principal payments on long-term debt and capitalized leases                   (615,000)          (515,000)
                                                                               ------------       ------------

     Net cash from financing activities                                            (628,000)        (1,307,000)
                                                                               ------------       ------------

     Net (decrease) in cash and cash equivalents                                   (205,000)        (2,342,000)

     Cash and cash equivalents at beginning of period                            12,903,000         11,114,000
                                                                               ------------       ------------

     Cash and cash equivalents at end of period                                $ 12,698,000       $  8,772,000
                                                                               ============       ============

SUPPLEMENTAL CASH FLOW DISCLOSURE:
     Cash paid during the period for:

          Interest                                                             $    586,000       $    201,000
                                                                               ============       ============

          Income taxes                                                         $      2,000       $  1,192,000
                                                                               ============       ============
</TABLE>


                             See accompanying notes

                                       4
<PAGE>   5


                        AMERICAN SHARED HOSPITAL SERVICES
              NOTES TO CONDENSED CONSOLIDATED FINANCIAL STATEMENTS
                                   (Unaudited)

NOTE 1.        BASIS OF PRESENTATION

In the opinion of management, the accompanying unaudited condensed consolidated
financial statements contain all adjustments (consisting of only normal
recurring accruals) necessary to present fairly American Shared Hospital
Services' consolidated financial position as of March 31, 2000 and the results
of its operations for the three month periods ended March 31, 2000 and 1999,
which results are not necessarily indicative of results on an annualized basis.
Consolidated balance sheet amounts as of December 31, 1999 have been derived
from audited financial statements. These financial statements include the
accounts of American Shared Hospital Services (the "Company") and its
wholly-owned subsidiaries: MMRI, Inc.; European Shared Medical Services Limited;
American Shared Radiosurgery Services; African American Church Health and
Economic Services, Inc.; ACHES Insurance Services, Inc.; OR21, Inc.; and the
Company's majority-owned subsidiary, GK Financing, LLC.

The Company through its majority-owned subsidiary, GK Financing, LLC, provided
Gamma Knife units to eight medical centers as of March 31, 2000 in Arkansas,
California, Connecticut, Massachusetts, Ohio, Texas and Wisconsin.

All significant intercompany accounts and transactions have been eliminated in
consolidation.

NOTE 2.        PER SHARE AMOUNTS

Per share information has been computed based on the weighted average number of
common shares and dilutive common share equivalents outstanding. For the three
months ended March 31, 2000 basic earnings per share was computed using
3,812,000 common shares and diluted earnings per share was computed using
5,505,000 common shares and equivalents. For the three months ended March 31,
1999 basic earnings per share was computed using 4,387,000 common shares and
diluted earnings per share was computed using 5,948,000 common shares and
equivalents.

ITEM 2.        MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION
               AND RESULTS OF OPERATIONS

Medical services revenues increased $532,000 for the three month period ended
March 31, 2000 from $1,566,000 for the three month period ended March 31, 1999.
The increase reflects increased utilization at Gamma Knife centers in operation
for longer than one year (25%) and the addition of four new Gamma Knife units.
This increase was mitigated by the sale at the end of its contractual term of a
mature Gamma Knife unit that operated in first quarter 1999. The Company had
eight Gamma Knife units in operation at March 31, 2000 compared to five at March
31, 1999.


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<PAGE>   6
Total costs of operations increased $243,000 for the three month period ended
March 31, 2000 from $471,000 for the three month period ended March 31, 1999.
Medical services payroll decreased $4,000 for the three month period ended March
31, 2000 compared to the same period in the prior year. The decrease is
attributable to the fact that the Company does not currently provide labor as a
component of its Gamma Knife services. Maintenance and supplies increased $2,000
for the three month period ended March 31, 2000 compared to the same period in
the prior year due to additional Gamma Knife units that have started contract
maintenance after the expiration of each unit's warranty period. This increase
was partially offset by the sale of the mature unit described above and its
contract maintenance costs, and by lower negotiated contract maintenance rates.
Depreciation and amortization increased $133,000 for the three month period
ended March 31, 2000 compared to the same period in the prior year due to the
net increase of three additional Gamma Knife units. Other operating costs
increased $112,000 for the three month period ended March 31, 2000 compared to
the same period in the prior year due to an increase in insurance, marketing and
personal property tax expenses related to the increase in Gamma Knife units, as
well as the addition of certain operating expenses of one Gamma Knife unit,
which started in September 1999. For the Gamma Knife unit that started in
September 1999, the Company receives all of the Gamma Knife revenues, exclusive
of physician fees, and is responsible for all operational costs of the Gamma
Knife unit.

Selling and administrative costs increased $122,000 for the three month period
ended March 31, 2000 from $426,000 for the three month period ended March 31,
1999 primarily due to increased marketing costs, administrative payroll costs,
building rent and investor relations costs.

Interest expense increased $246,000 for the three month period ended March 31,
2000 from $255,000 for the three month period ended March 31, 1999 due to a net
increase of three additional Gamma Knife units, all of which were financed with
long-term debt.

Interest and other income increased $85,000 for the three month period ended
March 31, 2000 from $119,000 for the three month period ended March 31, 1999
primarily due to higher cash balances and higher interest rates on invested cash
balances.

Minority interest increased $58,000 for the three month period ended March 31,
2000 from $90,000 for the three month period ended March 31, 1999 due to
increased profitability of GK Financing, LLC. Minority interest represents the
19% interest of GK Financing's minority ownership.

The Company recorded no income tax expense or benefit in the first quarter 2000
compared to a net income tax benefit of $45,000 for the first quarter 1999. The
Company does not expect to record any income tax expense for the current year
due to net operating loss carryforwards available for tax purposes.

The Company had net income of $391,000 ($0.10 per basic share) for the three
month period ended March 31, 2000 compared to net income of $488,000 ($0.11 per
basic share) in the same period in the prior year. The decrease was primarily
due to an income tax benefit of $45,000 recorded in first quarter 1999 and four
new Gamma Knife units being in operation for less than nine months and having
not yet reached the revenue and earnings potential of more mature

                                       6
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systems. In addition, as described above, a mature system that was in operation
during first quarter 1999 was sold in late 1999 at the end of its five year
contract term. The Company's operating margin (medical services revenues less
costs of operations) as a percentage of medical services revenues decreased from
70% in first quarter 1999 to 66% in first quarter 2000 due to a larger
proportion of more mature units being in operation in first quarter 1999
compared to first quarter 2000.

LIQUIDITY AND CAPITAL RESOURCES

The Company had cash and cash equivalents of $12,698,000 at March 31, 2000
compared to $12,903,000 at December 31, 1999. The Company's cash position
decreased $205,000 primarily due to an increase in accounts receivable
associated with increased revenues and the start-up cash collection phase of a
Gamma Knife unit in which the Company indirectly receives payment from insurers.

The Company as of March 31, 2000 had shareholders' equity of $13,017,000,
working capital of $11,181,000 and total assets of approximately $37,018,000.

The Company has scheduled interest and principal payments under its debt
obligations of approximately $5,095,000 during the next 12 months. The Company
believes that its cash flow from operations and cash resources is adequate to
meet its scheduled debt obligations during the next 12 months.

The Company is investing its cash in overnight repurchase agreements and
commercial paper pending use in the Company's operations.


                                       7
<PAGE>   8


                           PART II - OTHER INFORMATION

ITEM 1.        LEGAL PROCEEDINGS.
               None.

ITEM 2.        CHANGES IN SECURITIES.
               None.

ITEM 3.        DEFAULTS UPON SENIOR SECURITIES.
               None.

ITEM 4.        SUBMISSION OF MATTERS TO A VOTE OF SECURITIES HOLDERS.
               None.

ITEM 5.        OTHER INFORMATION.
               None.

ITEM 6.        EXHIBITS AND REPORTS ON FORM 8-K.

               (a)    Exhibits

                      The following exhibit is filed herewith:

                      Exhibit Number   Description
                      --------------   -----------
                      27               Financial Data Schedule

               (b)    Report on Form 8-K

                      Current report on Form 8-K dated January 14, 2000
                      reporting a change in the Company's independent
                      accountants.

                                       8
<PAGE>   9

                                   SIGNATURES

Pursuant to the requirements 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.

                        AMERICAN SHARED HOSPITAL SERVICES
                                   Registrant



Date:          May 12, 2000           /s/ Ernest A. Bates
                                      ------------------------------------------
                                      Ernest A. Bates, M.D.
                                      Chairman of the Board and
                                      Chief Executive Officer



Date:          May 12, 2000           /s/ Craig K. Tagawa
                                      ------------------------------------------
                                      Craig K. Tagawa
                                      Senior Vice President
                                      Chief Operating and Financial Officer

                                       9