<ARTICLE> 5
<LEGEND>
THIS SCHEDULE CONTAINS SUMMARY FINANCIAL INFORMATION EXTRACTED FROM COSTAR
GROUP, INC. CONSOLIDATED FINANCIAL STATEMENTS AND IS QUALIFIED IN ITS ENTIRETY
BY REFERENCE TO SUCH CONSOLIDATED FINANCIAL STATEMENTS AS OF AND FOR THE YEAR
ENDED DECEMBER 31, 1998 AND THE NINE MONTHS ENDED SEPTEMBER 30, 1999.
</LEGEND>
<CIK> 0001057352
<NAME> COSTAR GROUP
<MULTIPLIER> 1,000

                                                  
<PERIOD-TYPE>                   12-MOS                   9-MOS
<FISCAL-YEAR-END>                          DEC-31-1998             DEC-31-1999
<PERIOD-START>                             JAN-01-1998             JAN-01-1999
<PERIOD-END>                               DEC-31-1998             SEP-30-1999
<CASH>                                          19,667                  99,278
<SECURITIES>                                         0                       0
<RECEIVABLES>                                    1,572                   3,091
<ALLOWANCES>                                       326                     689
<INVENTORY>                                          0                       0
<CURRENT-ASSETS>                                21,238                 102,500
<PP&E>                                           3,385                   7,080
<DEPRECIATION>                                   1,228                   2,029
<TOTAL-ASSETS>                                  27,542                 132,561
<CURRENT-LIABILITIES>                            4,338                   9,478
<BONDS>                                              0                       0
<PREFERRED-MANDATORY>                                0                       0
<PREFERRED>                                          0                       0
<COMMON>                                            88                     129
<OTHER-SE>                                      23,115                 122,954
<TOTAL-LIABILITY-AND-EQUITY>                    27,542                 132,561
<SALES>                                              0                       0
<TOTAL-REVENUES>                                13,900                  21,326
<CGS>                                                0                       0
<TOTAL-COSTS>                                    4,562                   9,279
<OTHER-EXPENSES>                                     0                       0
<LOSS-PROVISION>                                 3,526                     363
<INTEREST-EXPENSE>                                 120                       0
<INCOME-PRETAX>                                (3,185)                 (9,962)
<INCOME-TAX>                                         0                       0
<INCOME-CONTINUING>                            (3,526)                 (8,049)
<DISCONTINUED>                                       0                       0
<EXTRAORDINARY>                                      0                       0
<CHANGES>                                            0                       0
<NET-INCOME>                                   (3,185)                 (8,049)
<EPS-BASIC>                                     (0.44)                  (0.71)
<EPS-DILUTED>                                   (0.44)                  (0.71)