<TABLE> <S> <C>

<PAGE>
<ARTICLE> 9
<LEGEND>
This schedule contains financial information extracted from the consolidated
financial statements of InterWest Bancorp. Inc. as of and for the six months
ended March 31, 1999 and is qualified in its entirety by reference to such
financial statements.
</LEGEND>
<MULTIPLIER> 1,000
       
<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          SEP-30-1999
<PERIOD-START>                             OCT-01-1998
<PERIOD-END>                               MAR-31-1999
<CASH>                                          65,421
<INT-BEARING-DEPOSITS>                          67,225
<FED-FUNDS-SOLD>                                14,439
<TRADING-ASSETS>                                     0
<INVESTMENTS-HELD-FOR-SALE>                    696,591
<INVESTMENTS-CARRYING>                          76,613
<INVESTMENTS-MARKET>                            76,078
<LOANS>                                      1,223,911
<ALLOWANCE>                                     13,506
<TOTAL-ASSETS>                               2,435,797
<DEPOSITS>                                   1,543,559
<SHORT-TERM>                                    14,126
<LIABILITIES-OTHER>                             20,658
<LONG-TERM>                                    685,304
<PREFERRED-MANDATORY>                                0
<PREFERRED>                                          0
<COMMON>                                        31,256
<OTHER-SE>                                     140,894
<TOTAL-LIABILITIES-AND-EQUITY>               2,435,797
<INTEREST-LOAN>                                 63,466
<INTEREST-INVEST>                               24,322
<INTEREST-OTHER>                                 1,950
<INTEREST-TOTAL>                                89,738
<INTEREST-DEPOSIT>                              28,166
<INTEREST-EXPENSE>                              48,146
<INTEREST-INCOME-NET>                           41,592
<LOAN-LOSSES>                                      998
<SECURITIES-GAINS>                                  73
<EXPENSE-OTHER>                                 33,576
<INCOME-PRETAX>                                 22,446
<INCOME-PRE-EXTRAORDINARY>                      14,731
<EXTRAORDINARY>                                      0
<CHANGES>                                            0
<NET-INCOME>                                    14,731
<EPS-PRIMARY>                                     0.94
<EPS-DILUTED>                                     0.92
<YIELD-ACTUAL>                                    3.59
<LOANS-NON>                                      8,116
<LOANS-PAST>                                         0
<LOANS-TROUBLED>                                     0
<LOANS-PROBLEM>                                      0
<ALLOWANCE-OPEN>                                13,224
<CHARGE-OFFS>                                      979
<RECOVERIES>                                       263
<ALLOWANCE-CLOSE>                               13,506
<ALLOWANCE-DOMESTIC>                             8,353
<ALLOWANCE-FOREIGN>                                  0
<ALLOWANCE-UNALLOCATED>                          5,153
        

</TABLE>