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                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

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

                                  FORM 10-K/A

(Mark one)

[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

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

     For the transition period from ________________ to __________________.

                           Commission File No. 0-27600

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

                          OPTICAL SENSORS INCORPORATED
             (Exact name of registrant as specified in its charter)

                   Delaware                              41-164359
       (State or other jurisdiction of               (I.R.S. Employer
        incorporation or organization)              Identification No.)

     7615 Golden Triangle Drive, Suite C
              Technology Park V
            Minneapolis, Minnesota                      55344-3733
   (Address of principal executive offices)             (Zip code)

       Registrant's telephone number, including area code: (952) 944-5857

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

           Securities registered pursuant to Section 12(g) of the Act:

                          Common Stock, $.01 par value
                         Preferred Share Purchase Rights

     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 [_]

     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. [_]

     As of March 15, 2000, 8,962,777 shares of Common Stock of the Registrant
were outstanding, and the aggregate market value of the Common Stock of the
Registrant as of that date (based upon the last reported sale price of the
Common Stock at that date as reported by the Nasdaq National Market System),
excluding outstanding shares beneficially owned by directors and executive
officers, was $27,832,268.

                       DOCUMENTS INCORPORATED BY REFERENCE

     Part III of this Annual Report on Form 10-K incorporates by reference
information (to the extent specific sections are referred to herein) from the
Registrant's Proxy Statement for its 2000 Annual Meeting of Stockholders to be
held on May 4, 2000 (the "2000 Proxy Statement").

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Item 6. SELECTED FINANCIAL DATA.

<TABLE>
<CAPTION>
                                                          Years Ended December 31,
                                     -------------------------------------------------------------------
                                       1999          1998          1997           1996           1995
                                     --------     ---------    -----------     ----------     ----------
                                                    (in thousands, except per share data)
<S>                                  <C>          <C>           <C>            <C>            <C>
Statements of Operations Data

Net sales.....................       $    134     $   1,019     $      141     $      163     $       --
Operating expenses............          5,968        10,533         10,472          9,734          8,249
Loss from operations..........         (7,879)      (12,420)       (12,527)       (10,941)        (8,249)
Interest income, net..........            101           628          1,193          1,555            118
Net loss......................         (7,785)      (11,817)       (11,333)        (9,385)        (8,131)
Net loss per common share,
  basic and diluted...........           (.88)        (1.34)         (1.35)         (1.30)        (19.27)


                                                                December 31,
                                     -------------------------------------------------------------------
                                       1999          1998          1997           1996           1995
                                     --------     ---------    -----------     ----------     ----------
                                                               (in thousands)
<S>                                  <C>          <C>           <C>            <C>            <C>
Balance Sheet Data

Cash and cash equivalents......        $ 1,451      $ 8,080        $17,101        $30,135       $ 5,395
Working capital................          2,001        9,103         18,220         30,039         5,242
Total assets...................          4,296       12,565         21,626         32,369         6,367
Long-term obligations..........            104          495            472             --            --
Total shareholders' equity.....          3,355       10,984         20,157         31,050         5,778
</TABLE>

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

Result of Operations

Fiscal Years Ended December 31, 1999 and 1998

     Net sales in 1999 decreased $884,733 or 87% to $134,131 from $1,018,864 in
1998. The decrease in sales is the result of the Company's suspension of direct
sales and support activities of the SensiCath System in January 1999. Net sales
for 1999 consisted of development fees and sales of SensiCath sensors to
existing customers, less OpticalCAM product returns. No new customer sales were
made during 1999. The Company does not expect meaningful sales of the SensiCath
System in the future.

     Costs of products sold in 1999 decreased $860,383 or 30% to $2,045,183 from
$2,905,566 in 1998. The decreased cost of products sold for 1999 was directly
related to the suspension of SensiCath production in January 1999. Costs of
products sold for 1999 included approximately $400,000 in fourth quarter
writeoffs of SensiCath production tooling, equipment and inventories. Remaining
fixed overhead costs and costs of personnel in support of CapnoProbe activities
and the Company's proprietary technologies are expected to continue at
approximately $250,000 per quarter through 2000.

     Research and development costs for 1999 decreased $1,132,954 or 27% to
$3,115,075 from $4,248,029 in 1998. The decrease for 1999 is due primarily to a
reduction in research and development staffing of approximately 25% in the first
quarter of 1999. Research and development efforts during 1999

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were directed towards product development and regulatory activities for the
CapnoProbe product. Research and development expenses are expected to be
approximately $500,000 per quarter through 2000. Under the July 1998 license
agreement with ICCM, the Company paid $300,000 in minimum royalties in 1999 and
expects to pay $300,000 in minimum royalties in 2000. The minimum royalty
payments paid in 1999 were recorded as research and development expenses because
no CapnoProbe sales took place in 1999. The Company is obligated to pay ICCM a
customary royalty equal to a percentage of sales, which varies depending on the
selling price to the customer of the CapnoProbe.

     Selling, general and administrative expenses in 1999 decreased $3,431,929
or 55% to $2,852,975 from $6,284,904 for 1998. Substantially all sales and
marketing activities were suspended during the first quarter of 1999, accounting
for the decrease from 1998. The Company expects selling, general and
administrative expenses to be approximately $450,000 per quarter through 2000,
not including expenses that might result from its activities in securing a
corporate merger, sale of a portion or all of the Company. Selling, general and
administrative expenses consist primarily of the cost of CapnoProbe marketing
clinical activities, ongoing administrative activities and costs of maintaining
the Company's public status.

     Net interest income in 1999 decreased $527,074 to $101,270 from $628,344 in
1998. The decrease in net interest income in 1999 is due to declining cash
balances. The Company expects interest income to continue to decline in future
periods as it uses cash for operations.

     Since its inception, the Company has experienced significant operating
losses. The Company incurred a net loss of $7,785,276 for 1999, compared to a
net loss of $11,817,330 for 1998. As of December 31, 1999, the Company had an
accumulated deficit of $66,151,326. The Company anticipates that its operating
losses will continue in the foreseeable future. Except for historical
information contained herein, the disclosures in this report are forward looking
statements. See "Certain Important Factors."

Fiscal Years Ended December 31, 1998 and 1997

     Net sales were $1,018,864 and $140,936 for 1998 and 1997, respectively.
Sales in 1997 were adversely affected by a recall of the SensiCath initiated by
the Company in May 1997 because of an interference problem with a certain
portion of the critical care patient population. In December 1997, the Company
introduced an enhanced version of its SensiCath Sensor that solved the inference
problems, accounting for the sales increase in 1998. In 1998, approximately 26%
of net sales were from the sale of SensiCath Sensors, and approximately 74% of
net sales were from the placement of OpticalCAM instrumentation. In January
1999, the Company discontinued direct sales activities of the SensiCath System
in order to focus its resources on development of the CapnoProbe product.

     Costs of products sold were $2,905,506 and $2,195,714 in 1998 and 1997,
respectively, an increase of $709,792 in 1998. The increase in 1998 was the
result of higher sales and manufacturing levels. A total of $446,000 in 1997
represented a write-down of OpticalCAM inventories to estimated market value.
The amount of the write-down reflected the difference between the Company's
estimated net realizable value based on the future selling price to its
customers of the OpticalCAM System and the cost of inventories on hand or on
order at the end of 1997. Under the agreement between the Company and IL, the
Company agreed to sell OpticalCAM instrumentation to IL at the lower of the
Company's direct cost of manufacturing or previously scheduled amounts.

     Research and development expenses were $4,248,029 and $4,975,037 in 1998
and 1997, respectively, a decrease of $727,008, or 15% in 1998. Research and
development expenses in 1997 included a $500,000 payment to Marquette Medical
Systems, Inc. under a previously disclosed

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technology purchase agreement. No comparable payments occurred in 1998. In 1998
the Company's research and development efforts were directed primarily towards
SensiCath System improvements. Towards the end of 1998, research and development
efforts were directed increasingly towards development of the CapnoProbe
product.

     Selling, general and administrative expenses were $6,284,904 and $5,496,772
in 1998 and 1997, respectively, an increase of $788,132, or 14%, in 1998. The
increase is attributable primarily to increased sales activities in 1998. The
Company's administrative expenses were essentially unchanged in 1998 from the
prior year. In January 1999, the Company discontinued direct sales activities.

     Net interest income decreased $618,318 to $628,344 in 1998 from $1,246,662
in 1997, due to declining cash reserves resulting from negative cash flows.

     The Company incurred a net loss of $11,817,330 in 1998 compared to a net
loss of $11,333,358 in 1997. The increase in net loss in 1998 was primarily due
to the decrease in net interest income described above. Increased spending in
selling expenses were offset by reductions in other areas.

Liquidity and Capital Resources

     To date, the Company has financed its operations primarily through the sale
of equity securities. From inception through December 31, 1995, the Company
raised net proceeds of $30,400,000 from private equity financings and stock
option exercises. In the first quarter of 1996, the Company completed an initial
public offering of 2,875,000 shares of Common Stock. The net proceeds to the
Company from the public offering were approximately $33,916,000. In January
1998, the Company sold 441,203 shares of Common Stock to IL, which represented
4.99% of the Company's outstanding Common Stock following completion of the
transaction, at a price of $5.00 per share (which is equal to the closing market
price on the date before signing of the agreement) for a total price of
$2,206,015.

     In March 2000, the Company entered into an Investment Agreement with Circle
F Ventures, LLC and Special Situations Fund III, L.P., pursuant to which the
Company agreed to issue convertible promissory notes in the aggregate principal
amount of up to $3,000,000. These notes are convertible into units, each unit
consisting of 50,000 shares of Common Stock and a five-year warrant to purchase
12,500 shares of Common Stock at an exercise price of $1.00 per share, at a
conversion price equal to $50,000 per unit, in accordance with the Investment
Agreement. The Company received advances under these notes in the aggregate
amount of $1,400,000 on March 10, 2000. The Company has the right to request
additional advances up to the aggregate principal amount of $1,600,000 at any
time during the 60 day period beginning on the first day after both of the
following have occurred: (1) the Company executes a definitive distribution
agreement for the Company's CapnoProbe product with a major medical company, and
(2) the stockholders of the Company have approved the conversion of any
additional advances to be made under the notes into units at the Company's 2000
Annual Meeting of Stockholders. The Company's right to request additional
advances expires on June 15, 2000.

     The proceeds from the sales of these securities have been used to fund
costs of producing products and for the operating expenses described above and
capital expenditures described below. The Company's Common Stock is quoted on
the Nasdaq National Market under the symbol "OPSI."

     The Company's cash and cash equivalents were $1,450,872 and $8,079,871 at
December 31, 1999 and 1998, respectively. The decrease in the Company's cash
balance is due to the operating losses described above. The Company incurred
cash expenditures of $6,413,007 for operations and $38,365 for capital
expenditures in 1999. In addition, the Company acquired equipment and tooling
under capital

                                       3
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leases for a total of $140,036 in 1999. The capital equipment expenditures were
principally for payments related to end of lease provisions under operating and
capital leases. The inventory at December 31, 1999 consisted primarily of
OpticalCAM monitors and ABG Modules for which the Company plans to use in
conjunction with future configurations of the CapnoProbe product and other
research and commercial applications.

     The Company has contracts to purchase minimum quantities of instrumentation
and other sole source inventory items with an outstanding aggregate commitment
of approximately $1,400,000 in 2000. The Company is currently negotiating
nullification of these obligations. It is the Company's opinion based on
discussions with the parties to these obligations that any future costs to the
Company under these agreements are unlikely. The Company is obligated to pay
ICCM $300,000 annually under the previously described license agreement. As of
December 31, 1999, the Company had no material commitments outstanding for
tooling, equipment or outside development contracts. The Company is currently in
discussion with its equipment lessor regarding prepayment of certain lease
obligations.

     The Company had previously reported that it was renegotiating an obligation
to prepay $238,393 in certain end of lease provisions due to underutilization of
a lease line. The negotiations resulted in this payment plus interest being
rescheduled into twelve monthly installments beginning January 2000.

     The Company believes that its current cash balances, including the
proceeds received on March 10, 2000 from advances under the notes issued under
the Investment Agreement described above, will be sufficient to fund the
Company's operations through June 30, 2000. Accordingly, the report of the
independent auditors on the Company's 1999 financial statements contains an
explanatory paragraph regarding the Company's ability to continue as a going
concern. Based on additional advances the Company expects to receive in 2000
under the Investment Agreement if the Company signs a definitive distribution
agreement for its CapnoProbe product and the Company's stockholders approve the
conversion of any additional advances to be made under the notes into units at
the Company's 2000 Annual Meeting of Stockholders and payments the Company
expects to receive in 2000 if it signs a definitive distribution agreement for
its CapnoProbe product, the Company believes that it will have sufficient cash
to fund its operations through 2000. There can be no assurance, however, that
the Company will enter into a definitive distribution agreement for its
CapnoProbe product, obtain the approval of the conversion of the notes into
units by the Company's stockholders or otherwise obtain additional financing on
satisfactory terms, or at all. In addition, the Company has based its estimates
of how long its cash balances will last on assumptions that may prove to be
wrong. If the Company is unable to obtain additional financing when needed, it
will likely be forced to cease operations.

Certain Important Factors

     In addition to the factors identified above, there are several important
factors that could cause the Company's actual results to differ materially from
those anticipated by the Company or which are reflected in any forward-looking
statements of the Company. These factors, and their impact on the success of the
Company's operations and its ability to achieve its goals, include the
following:

     o    Need for Additional Financing. The Company believes that its current
          cash balances, including the proceeds received on March 10, 2000 from
          advances under the notes issued under the Investment Agreement
          described above, will be sufficient to fund the Company's operations
          through June 30, 2000. Accordingly, the report of the independent
          auditors on the Company's 1999 financial statements contains an
          explanatory paragraph regarding the Company's ability to continue as
          a going concern. Based on additional advances the Company expects to
          receive in 2000 under the Investment Agreement if the Company signs

                                       4
<PAGE>

          a definitive distribution agreement for its CapnoProbe product and the
          Company's stockholders approve the conversion of any additional
          advances to be made under the notes into units at the Company's 2000
          Annual Meeting of Stockholders and payments the Company expects to
          receive in 2000 if it signs a definitive distribution agreement for
          its CapnoProbe product, the Company believes that it will have
          sufficient cash to fund its operations through 2000. There can be no
          assurance, however, that the Company will enter into a definitive
          distribution agreement for its CapnoProbe product, obtain the
          approval of the conversion of the notes into units by the Company's
          stockholders or otherwise obtain additional financing on satisfactory
          terms, or at all. In addition, the Company has based its estimates of
          how long its cash balances will last on assumptions that may prove to
          be wrong. If the Company is unable to obtain additional financing
          when needed, it will likely be forced to cease operations.
          Additionally, any additional equity financings may be dilutive to the
          Company's existing stockholders, and debt financing, if available, may
          involve restrictive covenants on the Company's business.

     o    Development and Commercialization of CapnoProbe. The Company's future
          success will depend, in large part, on successful development and
          commercialization of the CapnoProbe product. The Company is in the
          later stages of developing and testing prototypes and is currently
          engaged in human clinical trials of the CapnoProbe product. The
          Company has set up one manufacturing pod for manual assembly of the
          prototype probes and finished preliminary plans for automated probe
          assembly. The Company projects that the product will be available for
          limited release in 2000 and full commercial release in the first
          quarter of 2001. The Company has not yet established commercial
          manufacturing for the CapnoProbe. Accordingly, there can be no
          assurance that the Company will successfully develop a commercial
          CapnoProbe product.

     o    Completion of Corporate Alliance or Business Combination. In January
          1999, the Company announced that it had engaged Volpe Brown Whelan &
          Company, LLC, to serve as financial advisor to the Company. The
          Company continues to explore strategic alternatives, including joint
          ventures, corporate strategic alliances, sale of the business or
          product lines, or other business combinations. In January 2000, the
          Company signed a non-binding letter of intent with a major supplier of
          medical products and services to negotiate a definitive agreement for
          the Company's CapnoProbe product. There can be no assurance that the
          Company will be able to enter into a definitive distribution agreement
          for its CapnoProbe product with this party or otherwise complete a
          transaction with terms favorable to the Company.

     o    Nasdaq Listing Requirements. The Company's Common Stock is currently
          quoted on the Nasdaq National Market under the symbol "OPSI." In order
          to be listed on the Nasdaq National Market, the Company must maintain
          total net tangible assets of at least $4.0 million. As of December 31,
          1999, the Company had total net tangible assets of approximately $2.8
          million. On March 10, 2000, the Company entered into an Investment
          Agreement with two of the Company's principal stockholders, Circle F
          Ventures, LLC, a Georgia limited liability company, and Special
          Situations Fund III, L.P., a Delaware limited partnership, pursuant to
          which the Company agreed to issue convertible promissory notes in the
          aggregate principal amount of up to $3.0 million. The Company received
          advances under the notes in the aggregate amount of $1.4 million on

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          March 10, 2000. The $1.4 million received by the Company, however,
          will not count towards the $4.0 million net tangible asset requirement
          until the amount is converted into equity. The Company has the right
          to request additional advances up to the aggregate principal amount of
          $1.6 million at any time during the 60 day period beginning on the
          first day after both of the following have occurred: (1) the Company
          executes a definitive distribution agreement for the Company's
          CapnoProbe product with a major medical company, and (2) the
          stockholders of the Company have approved the conversion of any
          additional advances to be made under the notes into units at the
          Company's 2000 Annual Meeting of Stockholders. The Company's right to
          request additional advances will expire on June 15, 2000.

          In addition to the net tangible asset requirement, the closing bid
          price for the Company's Common Stock cannot be less than $1.00 per
          share for 30 consecutive days. The closing bid price for the Company's
          Common Stock has been less than $1.00 per share on several occasions
          within the last year, but never for 30 or more consecutive days. If,
          in the future, the Company had less than $4.0 million in total net
          tangible assets but more than $2.0 million in total net tangible
          assets, the Company's Common Stock would be eligible for quotation on
          the Nasdaq Small Cap Market, provided that the $1.00 minimum bid price
          requirement was met. If the Common Stock was not eligible for either
          the Nasdaq National Market or the Nasdaq Small Cap Market, it would
          likely be quoted in the "over-the-counter" market and eligible to
          trade on the OTC bulletin board. If the Company's Common Stock traded
          on the OTC bulletin board, trading, if any, would be subject to the
          "penny stock" rules under the Securities Exchange Act of 1934 as
          amended, and the public trading market for the Company's Common Stock
          could be adversely affected.

     o    Competition. Competition among medical device companies is intense and
          increasing. There can be no assurance that the Company's competitors
          will not succeed in developing or marketing technologies and products
          that are more effective or less expensive than the Company's products
          or that would render the Company's products obsolete or non-
          competitive.

     o    Regulatory Approvals. The Company's ability to market its current
          products and any products that it may develop in the future requires
          clearances or approvals from the FDA and other governmental agencies,
          including, in some instances, foreign and state agencies. The process
          for maintaining and obtaining necessary regulatory clearances and
          approvals can be expensive and time consuming. There can be no
          assurance that the Company will be able to maintain or obtain
          necessary regulatory approvals and clearances in the future.


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                                   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.

                                       OPTICAL SENSORS INCORPORATED


Dated: April 14, 2000                  By:   /s/ Paulita M. LaPlante
                                          -------------------------------------
                                          Paulita M. LaPlante
                                          President and Chief Executive Officer


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