SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM 10-K/A

For the fiscal year ended December 31, 1999

Commission file number 0-4804

TENNANT COMPANY
Incorporated in the State of Minnesota            Employer Identification Number 41-0572550

701 North Lilac Drive, P.O. Box 1452, Minneapolis, Minnesota 55440

Telephone Number 763-540-1208

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

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

Common Stock, par value $.375 per share

and

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. /x/

    $283,601,511.50 is aggregate market value of common stock held by non-affiliates as of March 6, 2000.

9,039,092 shares outstanding at March 6, 2000

DOCUMENTS INCORPORATED BY REFERENCE

    2000 Proxy—Part III (Partial)

(3) Investments as Lessor

    Through first quarter 1998, the Company leased floor maintenance equipment to customers under sales-type and operating leases. Noncancelable terms for sales-type leases ranged from six months to five years, and terms for operating leases range from one month to five years. All leases provide for minimum lease payments and require the lessees to pay executory costs. In 1998, the Company sold the portfolio to a third party with recourse and accordingly under SFAS 125 is required to report lease payments outstanding as an asset.

Minimum future lease payments to be received during the years ended December 31 are as follows:

    The Company's investment in equipment related to operating leases as of December 31 is as follows:

    The Company's net investment in sales-type leases at December 31 is as follows:

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