<ARTICLE> 5
<LEGEND>
The schedule contains summary financial information extracted from the condensed
consolidated balance sheet and statement of operations of COSTAR GROUP, INC. as
of and for the nine months ended September 30, 1999 and is qualified in its
entirety by reference to such financial statements.
</LEGEND>

                             
<PERIOD-TYPE>                   9-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               SEP-30-1999
<CASH>                                      99,278,000
<SECURITIES>                                         0
<RECEIVABLES>                                3,091,000
<ALLOWANCES>                                   689,000
<INVENTORY>                                          0
<CURRENT-ASSETS>                           102,500,000
<PP&E>                                       7,080,000
<DEPRECIATION>                               2,029,000
<TOTAL-ASSETS>                             132,561,000
<CURRENT-LIABILITIES>                        9,478,000
<BONDS>                                              0
<PREFERRED-MANDATORY>                                0
<PREFERRED>                                          0
<COMMON>                                       129,000
<OTHER-SE>                                 122,954,000
<TOTAL-LIABILITY-AND-EQUITY>               132,561,000
<SALES>                                              0
<TOTAL-REVENUES>                            21,326,000
<CGS>                                                0
<TOTAL-COSTS>                                9,279,000
<OTHER-EXPENSES>                            22,009,000
<LOSS-PROVISION>                               363,000
<INTEREST-EXPENSE>                                   0
<INCOME-PRETAX>                            (9,962,000)
<INCOME-TAX>                                         0
<INCOME-CONTINUING>                        (8,049,000)
<DISCONTINUED>                                       0
<EXTRAORDINARY>                                      0
<CHANGES>                                            0
<NET-INCOME>                               (8,049,000)
<EPS-BASIC>                                     (0.71)
<EPS-DILUTED>                                   (0.71)