<PAGE>   1

                       SECURITIES AND EXCHANGE COMMISSION

                             Washington, D.C. 20549

                                    FORM 10-Q

                  QUARTERLY REPORT UNDER SECTION 13 OR 15(d) OF
                       THE SECURITIES EXCHANGE ACT OF 1934


For quarter ended September 30,1999              Commission File Number 0-13147
                  -----------------                                     --------

                                   LESCO, INC.
--------------------------------------------------------------------------------
             (Exact name of registrant as specified in its charter)

         OHIO                                         34-0904517
-------------------------------         ---------------------------------------
State or other jurisdiction of          (I.R.S. Employer Identification Number)
incorporation or organization)

           20005 Lake Road
           Rocky River, Ohio                             44116
---------------------------------------                ----------
(Address of principal executive offices)               (Zip Code)

                                 (440) 333-9250
                         -------------------------------
                         (Registrant's telephone number,
                              including area code)


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 twelve 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 the number of shares outstanding of each of the issuer's classes of
common stock, as of the close of the latest practical date.

                                                         Outstanding at
                     Class                               November 11, 1999
        --------------------------------                 -----------------
        Common shares, without par value                 8,444,763 shares




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                                   LESCO, INC.
                           CONSOLIDATED BALANCE SHEETS

<TABLE>
<CAPTION>
                                                                                September 30    September 30     December 31
(In thousands except share data)                                                    1999            1998             1998
                                                                                ------------   -------------     -----------
                                                                                        (unaudited)
<S>                                                                               <C>             <C>             <C>
ASSETS

CURRENT ASSETS:
     Cash                                                                         $   2,937       $   3,044       $   1,841
     Accounts receivable -- net                                                      81,067          79,273          65,358
     Inventories                                                                    115,487          94,095          86,662
     Deferred income taxes                                                            1,727           1,993           1,568
     Prepaid expenses and other assets                                                2,453           1,725           3,598
                                                                                  ---------       ---------       ---------
         TOTAL CURRENT ASSETS                                                       203,671         180,130         159,027

Property, Plant and Equipment                                                        80,091          66,044          67,874
     Less allowance for depreciation and amortization                               (33,691)        (27,204)        (28,796)
                                                                                  ---------       ---------       ---------
                                                                                     46,400          38,840          39,078

Other Assets                                                                          8,899           9,909           9,643
                                                                                  ---------       ---------       ---------

         TOTAL ASSETS                                                             $ 258,970       $ 228,879       $ 207,748
                                                                                  =========       =========       =========

LIABILITIES AND SHAREHOLDERS' EQUITY

CURRENT LIABILITIES:
     Accounts payable                                                             $  54,017       $  50,688       $  36,138
     Other current liabilities                                                       12,540           8,283           6,688
     Current portion of  debt                                                           100             200             100
                                                                                  ---------       ---------       ---------
         TOTAL CURRENT LIABILITIES                                                   66,657          59,171          42,926

Long-term debt                                                                       99,021          86,770          83,698
Deferred income taxes                                                                 1,650           2,282           2,427

SHAREHOLDERS' EQUITY:
     Preferred shares-- without par value--
         authorized 500,000 shares
     Common shares--without par value--
         19,500,000 shares authorized; 8,507,488 shares issued and 8,444,763
         outstanding at September 30, 1999, 8,401,418
         at September 30, 1998, 8,366,202 at December 31, 1998                          851             841             841
     Paid-in capital                                                                 33,004          31,158          31,631
     Retained earnings                                                               58,922          49,588          47,156
     Less treasury shares                                                               (59)            (59)            (59)
     Unearned compensation                                                           (1,076)           (872)           (872)
                                                                                  ---------       ---------       ---------

         TOTAL SHAREHOLDERS' EQUITY                                                  91,642          80,656          78,697
                                                                                  ---------       ---------       ---------

         TOTAL LIABILITIES AND
         SHAREHOLDERS' EQUITY                                                     $ 258,970       $ 228,879       $ 207,748
                                                                                  =========       =========       =========
</TABLE>




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                                   LESCO, INC.
                              STATEMENTS OF INCOME

<TABLE>
<CAPTION>
                                                Three Months Ended September 30        Nine Months Ended September 30
                                                -------------------------------        ------------------------------

     (In thousands except per share data)            1999               1998               1999               1998
                                                  ---------          ---------          ---------          ---------
                                                          (unaudited)                           (unaudited)

<S>                                               <C>                <C>                <C>                <C>
Net sales                                         $ 130,938          $ 118,837          $ 364,707          $ 325,973

Cost of sales                                        86,563             81,996            240,464            218,880
                                                  ---------          ---------          ---------          ---------

     GROSS PROFIT ON SALES                           44,375             36,841            124,243            107,093

Warehouse & delivery expense                          9,312              9,263             28,356             25,779
Selling, general & administrative expense            24,715             22,441             72,061             64,729
                                                  ---------          ---------          ---------          ---------
                                                     34,027             31,704            100,417             90,508
                                                  ---------          ---------          ---------          ---------

     INCOME FROM OPERATIONS                          10,348              5,137             23,826             16,585

Other deductions (income):
     Interest expense                                 1,561              1,153              4,504              4,197
     Joint venture loss                                 573                909                875              1,699
     Other - net                                     (1,285)            (1,043)            (2,790)            (2,957)
                                                  ---------          ---------          ---------          ---------
                                                        849              1,019              2,589              2,939
                                                  ---------          ---------          ---------          ---------

Income Before Income Taxes                            9,499              4,118             21,237             13,646

Income taxes                                          3,702              1,605              8,280              5,321
                                                  ---------          ---------          ---------          ---------


     NET INCOME                                   $   5,797          $   2,513          $  12,957          $   8,325
                                                  =========          =========          =========          =========



     BASIC EARNINGS PER SHARE                     $    0.69          $    0.30          $    1.54          $    1.00
                                                  =========          =========          =========          =========

     DILUTED EARNINGS PER SHARE                   $    0.68          $    0.30          $    1.51          $    0.97
                                                  =========          =========          =========          =========
</TABLE>



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                                   LESCO, INC.
                      CONSOLIDATED STATEMENTS OF CASH FLOWS


<TABLE>
<CAPTION>
                                                                       Nine Months Ended
                                                                         September 30
                                                                 ----------------------------
(In thousands)                                                      1999               1998
                                                                 ---------          ---------
                                                                         (unaudited)
<S>                                                              <C>                <C>
OPERATING ACTIVITIES:
     Net income                                                  $  12,957          $   8,325
     Adjustments to reconcile net income to net cash
         (used) provided  by operating activities:
         Depreciation and amortization                               4,993              4,065
         Increase in accounts receivable                           (18,189)           (15,044)
         Provision for uncollectible accounts receivable             2,480              1,890
         Increase in inventories                                   (28,825)           (10,727)
         Increase in accounts payable                               17,879             16,466
         Increase in other current items                             6,838              5,052
         Other                                                         (33)              (743)
                                                                 ---------          ---------

     NET CASH (USED) PROVIDED BY
     OPERATING ACTIVITIES                                           (1,900)             9,284

INVESTING ACTIVITIES:
     Purchase of property, plant and equipment - net               (12,315)            (9,686)
     Acquisition of businesses                                           -             (8,174)
     Bond proceeds held for construction                                 -              4,761
                                                                 ---------          ---------

     NET CASH USED BY INVESTING ACTIVITIES                         (12,315)           (13,099)

FINANCING ACTIVITIES:
     Proceeds from borrowings                                      167,400            153,097
     Reduction of borrowings                                      (152,077)          (149,680)
     Issuance of common shares                                       1,172              1,122
     Cash dividend                                                  (1,184)            (1,084)
                                                                 ---------          ---------

     NET CASH PROVIDED BY FINANCING ACTIVITIES                      15,311              3,455
                                                                 ---------          ---------

Net increase (decrease) in cash                                      1,096               (359)

Cash - Beginning of the period                                       1,841              3,403
                                                                 ---------          ---------

     CASH - END OF THE PERIOD                                    $   2,937          $   3,044
                                                                 =========          =========
</TABLE>



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<PAGE>   5


                                   LESCO, INC.
                          NOTES TO FINANCIAL STATEMENTS
                          -----------------------------


NOTE A - Basis of Presentation
------------------------------

The accompanying unaudited financial statements have been prepared in accordance
with generally accepted accounting principles for interim financial information
and with the requirements of Regulation S-X and Form 10-Q. The statements
reflect all adjustments, consisting only of normal recurring accruals, which
are, in the opinion of management, necessary for a fair presentation of the
results for interim periods. For further information, refer to the audited
financial statements and footnotes thereto for the year ended December 31, 1998
included in the Company's Form 10-K.

Operating results for the nine months ended September 30 are not necessarily
indicative of the results to be expected for the year due to the seasonal nature
of the Company's business.

NOTE B - Earnings per Share
---------------------------

The following table sets forth the computation of basic and diluted earnings per
share:

<TABLE>
<CAPTION>
                                                Three Months Ended                   Nine Months Ended
                                                   September 30                        September 30
----------------------------------------------------------------------------------------------------------------
(In thousands except share data)              1999               1998              1999               1998
----------------------------------------------------------------------------------------------------------------

<S>                                       <C>                <C>                <C>                <C>
Numerator:
   Net Income                             $    5,797         $    2,513         $   12,957         $    8,325
Denominator:
   Denominator for basic
     earnings per share -
     weighted average shares               8,436,974          8,341,510          8,403,994          8,308,997

   Effect of dilutive securities:
     Employee stock options                  127,825            140,570            161,226            284,385
     Performance shares                                          18,301                                18,301
                                          ----------         ----------         ----------         ----------
   Diluted potential common
     Shares                                  127,825            158,871            161,226            302,686
   Denominator for diluted
     earnings per share -
     adjusted weighted average
     shares and assumed
     conversions                           8,564,799          8,500,381          8,565,220          8,611,683
Earnings per share
   Basic                                  $     0.69         $     0.30         $     1.54         $     1.00
   Diluted                                $     0.68         $     0.30         $     1.51         $     0.97
</TABLE>


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<PAGE>   6



                                   LESCO, INC.
                  FINANCIAL CONDITION AND RESULTS OF OPERATIONS
                      MANAGEMENT'S DISCUSSION AND ANALYSIS
                      ------------------------------------


Results of Operations
---------------------

For the third quarter ended September 30, 1999, sales increased 10.2% to $130.9
million from $118.8 million in 1998. Sales for the first nine months of 1999
increased 11.9% to $364.7 million from $326.0 million in 1998. Sales volumes for
both consumable and hard goods increased in the third quarter and the first nine
months of 1999 compared to 1998. Same store sales for the third quarter and the
first nine months of 1999 compared to 1998 increased 7.0% and 10.6%,
respectively.

Gross profit, as a percent of sales, for the third quarter ended September 30,
1999 was 33.9% compared to 31.0% in the third quarter of 1998. For the first
nine months, gross profit, as a percent of sales, in 1999 was 34.1% compared to
32.9% in 1998. Gross profit increases occurred in all major product categories
for the third quarter and the first nine months ended September 30. The increase
in gross profit, as a percent of sales, is due primarily to more cost effective
purchasing of raw materials and the positive effect of a favorable product mix.

The Company's warehouse and delivery expenses decreased as a percent of sales
for the third quarter from 7.8% in 1998 to 7.1% in 1999 despite a 10.2% sales
increase. For the nine months ended warehouse and delivery expenses decreased as
a percent of sales from 7.9% in 1998 to 7.8% in 1999 despite an 11.9% sales
increase. This is due primarily to the efficiencies gained by the warehouse
consolidation that occurred in 1998.

The Company's selling, general and administrative expenses increased $2.3
million, a 10.1% increase to $24.7 million for the third quarter of 1999
compared to $22.4 million in the third quarter of 1998. Selling, general and
administrative expenses remained unchanged as a percent of sales compared to the
third quarter a year ago. For the nine months ended selling, general and
administrative expenses increased $7.3 million, an 11.3% increase to $72.1
million for 1999 compared to $64.7 million in 1998. Selling, general and
administrative expenses as a percent of sales decreased 0.1% compared to a year
ago. The expense increases for both the third quarter and the first nine months
were primarily attributable to an increase in sales and administrative support
personnel added in the first half of 1999 to support the continuing business
growth.

Interest expense for the third quarter of 1999 increased to $1.6 million from
$1.2 million in 1998. For the first nine months interest expense increased to
$4.5 million in 1999 from $4.2 million in 1998. The third quarter and
year-to-date increase was primarily due to an increase in the level of
outstanding debt and higher interest rates in 1999 compared to 1998.

Other deductions-net include customer finance charges which totaled $1.0 million
in the third quarter of 1999 compared to $1.1 million in 1998 and year-to-date
of $2.3 million in 1999 and $2.4 million in 1998. The slight decrease is
primarily due to the impact of outsourcing the Company's equipment financing
programs.


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<PAGE>   7



The Company recognized the net operating loss of Commercial Turf Products, Ltd.,
the Company's 50/50 commercial equipment joint venture with MTD Products Inc of
$573,000 in the third quarter of 1999 compared to a net operating loss of
$909,000 in the third quarter of 1998. For the first nine months the Company
recognized a net operating loss of $875,000 in 1999 compared to a net operating
loss of $1.7 million in 1998. The improvement in performance is due primarily to
greater efficiencies resulting from the increase in the number of units produced
compared to a year ago.



Financial Condition, Liquidity and Capital Resources
----------------------------------------------------

As of September 30, 1999, total assets of the Company were $259.0 million
compared to $228.9 million as of September 30, 1998 and $207.7 million as of
December 31, 1998. The asset increase from September 30, 1998 to September 30,
1999 is primarily related to working capital increases and increases in
property, plant and equipment, while the working capital increase from December
31, 1998 is due primarily to seasonality. Net accounts receivable were $81.1
million as of September 30, 1999 compared to $79.3 million as of September 30,
1998 and $65.4 million as of December 31, 1998. Compared to a year ago, net
accounts receivable increased 2.3% compared to a 11.9% sales increase, due
primarily to the impact of outsourcing equipment financing and the continued
focus on the overall management of receivables. The increase in net accounts
receivable from December 31, 1998 to September 30, 1999 reflect the business
seasonality. Inventories were $115.5 million as of September 30, 1999 compared
to $94.1 million as of September 30, 1998 and $86.7 million as of December 31,
1998. The increase in inventory is due primarily to higher turfseed inventory
levels as a result of higher than anticipated turfseed crop yields and the
Company's new approach being utilized to market equipment.

Funding for the asset increases was provided primarily by an increase in
long-term debt, along with an increase in accounts payable. The Company's
long-term debt increased to $99.0 million as of September 30, 1999 compared to
$86.8 million as of September 30, 1998 and $83.7 million as of December 31,
1998. The increase from September 30, 1998 and December 31, 1998 to September
30, 1999 was primarily due to increased inventory levels as noted above. The
increase in accounts payable from September 30, 1998 to September 30, 1999
relates primarily to the increases in inventory, while the increase from
December 31, 1999 is primarily related to seasonality.

Outstanding debt under the Company's credit facility was $42.8 million as of
September 30, 1999 compared to $23.0 million as of September 30, 1998 and $19.9
million as of December 31, 1998. As of September 30, 1999 the Company had $17.2
million available under its bank credit facility. The Company completed the
renewal of its revolving credit agreement, which provides $60.0 million of
unsecured financing with a maturity of June 30, 2003 and believes its current
borrowing capacity is adequate for the foreseeable future.

Capital expenditures for the first nine months of 1999 included improvements in
the Company's information systems, construction costs for the Company's new
fertilizer plant in Sebring, Florida, investment in the new fertilizer
technology, and general improvements to the Company's other manufacturing and
distribution facilities.



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<PAGE>   8



Year 2000 Compliance
--------------------

The Year 2000 issue concerns the potential inability of computer hardware or
software to properly recognize date-sensitive information relating to the Year
2000 and beyond. To address this potential problem, the Company has implemented
a work plan to identify information systems issues and has surveyed each
manufacturing location to identify mission-critical and support-function system
compliance issues. The Company has assessed potential risks that could impact
the Company's business and is developing appropriate contingency plans in the
event of a temporary disruption. The Company's Vice President, Chief Information
Officer leads this effort.

Since 1995, the Company has conducted a broad-based information systems program
to replace and upgrade computer application software and hardware in its
manufacturing, distribution, point-of-sale, financial and administrative
functions. The primary purpose of this program is to improve operating
efficiencies. With the implementation of a new receivables management system in
1999, the Company expects to complete this broad-based information systems
program by the end of 1999. At the time of purchase of these new systems from
vendors, the Company required that these systems be Year-2000 compliant. The
Company has obtained written vendor representations as to the new systems' Year
2000 compliance. It is the Company's belief that these new information systems
and the related hardware to operate these systems are compliant with Year 2000
computing requirements.

During 1998, the Company completed construction of a new fertilizer
manufacturing plant in Sebring, Florida. It is the Company's belief that all
operating systems at this plant are Year-2000 compliant. During 1998, the
Company conducted a review of each manufacturing site, and internal assessments
with respect to Year 2000 compliance for its desktop and manufacturing process
technology and other potentially date-sensitive technology. The Company has
implemented a work plan to ensure all systems are compliant by the end of 1999.
The Company has been testing and will continue to test Year 2000 compliance
throughout its operations during 1999. In addition, the Company is communicating
with key business partners including suppliers, utilities and customers to
determine their risks associated with the Year 2000.

The primary focus of the Company's broad-based information systems upgrades has
been to improve operating efficiencies; however, Year 2000 compliance has been a
secondary benefit of this initiative. The Company has funded these system
improvements from capital funds and annual operating cash flows. The incremental
cost specifically associated with Year 2000 compliance efforts has been nominal,
and is expected to be less than $150,000 in 1999.

The potential risks associated with the Year 2000 issue include temporary
disruption of manufacturing operations, materials ordering, receiving and
shipping product, order entry, billing and collection of accounts receivable,
and disruption in services from vendors who supply materials or services
necessary to the Company in the conduct of its operations. While there can be no
complete assurances, we believe the risk of disruption to the Company has been
minimized as a result of the information systems upgrades that have occurred
since 1995, the ongoing testing and assessments of operating systems, and the
ongoing communication with key business partners. However, if disruptions
relating to the Year 2000 issue occur, the Company could experience some adverse
effects on its operations. The Company is developing contingency plans for
various functional areas, and anticipates completion of these plans by November
30, 1999. During 1999, the Company will continue to assess risks to minimize the
impact of any potential disruptions.



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Forward-Looking Statements
--------------------------

Certain statements included in the report are forward-looking statements that
are based on management's current belief, assumptions and expectations. These
forward-looking statements can be identified by the use of predictive or future
tense terms such as "anticipate," "estimate," "project," "may," "will" or
similar terms. These statements are made pursuant to the safe harbor provisions
of the Private Securities Litigation Reform Act of 1995. The Company's actual
future performance may differ materially from that anticipated in
forward-looking statements. Risk factors that would cause or contribute to such
differences include, but are not limited to:

         -    regional weather conditions which have an impact on both timing
              and volume of sales;
         -    the Company's successful execution of its operating plans;
         -    the Company's ability to integrate business acquisitions
              successfully;
         -    general economic and business conditions;
         -    changes in market demographics; and
         -    changes in the regulation of the Company's products, including
              applicable environmental regulations.



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<PAGE>   10



                           PART II - OTHER INFORMATION
                           ---------------------------

Except as noted below, the items in Part II are inapplicable or, if applicable,
would be answered in the negative. These items have been omitted and no other
reference is made thereto.


Item 6 - Exhibits and Reports on Form 8-K
-----------------------------------------

(a)      Exhibits:

         (10)(v) Credit Agreement dated September 23, 1999 among National City
         Bank, PNC Bank, National Association, Bank One, Michigan, National City
         Bank, as agent and the Registrant

        (27) Financial Data Schedule


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<PAGE>   11





                                   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.

                                            LESCO, INC.





November 12, 1999                           By:    /s/ R. Breck Denny
-----------------                               --------------------------------
                                            R. Breck Denny, Vice-President/
                                                  Chief Financial Officer




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