<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, 1998 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-30-1998
<PERIOD-START>                             OCT-01-1997
<PERIOD-END>                               MAR-31-1998
<CASH>                                          39,285
<INT-BEARING-DEPOSITS>                          40,651
<FED-FUNDS-SOLD>                                     0
<TRADING-ASSETS>                                     0
<INVESTMENTS-HELD-FOR-SALE>                    600,277
<INVESTMENTS-CARRYING>                          93,739
<INVESTMENTS-MARKET>                            91,973
<LOANS>                                      1,211,065
<ALLOWANCE>                                      9,268
<TOTAL-ASSETS>                               2,091,022
<DEPOSITS>                                   1,238,725
<SHORT-TERM>                                   500,564
<LIABILITIES-OTHER>                             14,497
<LONG-TERM>                                    195,880
<PREFERRED-MANDATORY>                                0
<PREFERRED>                                          0
<COMMON>                                         1,696
<OTHER-SE>                                     139,660
<TOTAL-LIABILITIES-AND-EQUITY>               2,091,022
<INTEREST-LOAN>                                 53,067
<INTEREST-INVEST>                               23,590
<INTEREST-OTHER>                                   782
<INTEREST-TOTAL>                                77,439
<INTEREST-DEPOSIT>                              26,648
<INTEREST-EXPENSE>                              46,271
<INTEREST-INCOME-NET>                           31,168
<LOAN-LOSSES>                                      639
<SECURITIES-GAINS>                                 323
<EXPENSE-OTHER>                                 25,132
<INCOME-PRETAX>                                 15,480
<INCOME-PRE-EXTRAORDINARY>                      10,051
<EXTRAORDINARY>                                      0
<CHANGES>                                            0
<NET-INCOME>                                    10,051
<EPS-PRIMARY>                                     1.19
<EPS-DILUTED>                                     1.17
<YIELD-ACTUAL>                                    3.31
<LOANS-NON>                                      6,731
<LOANS-PAST>                                         0
<LOANS-TROUBLED>                                     0
<LOANS-PROBLEM>                                      0
<ALLOWANCE-OPEN>                                 9,033
<CHARGE-OFFS>                                      577
<RECOVERIES>                                       192
<ALLOWANCE-CLOSE>                                9,268
<ALLOWANCE-DOMESTIC>                             5,340
<ALLOWANCE-FOREIGN>                                  0
<ALLOWANCE-UNALLOCATED>                          3,928
        

</TABLE>