WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.
<TABLE> <S> <C>
<ARTICLE> 6
<RESTATED>
<CIK> 0000882912
<NAME> NUVEEN TAX-FREE UNIT TRUST
<SERIES> Series 691
<NAME> MASSACHUSETTS INSURED TRUST
<NUMBER> 105
<MULTIPLIER> 1
<CURRENCY> U.S. DOLLARS
<FISCAL-YEAR-END> JUN-30-2000
<PERIOD-START> JUL-01-1999
<PERIOD-END> JUN-30-2000
<PERIOD-TYPE> YEAR
<EXCHANGE-RATE> 1
<INVESTMENTS-AT-COST> 4,178,643
<INVESTMENTS-AT-VALUE> 4,126,573
<RECEIVABLES> 93,025
<ASSETS-OTHER> 0
<OTHER-ITEMS-ASSETS> 0
<TOTAL-ASSETS> 4,219,598
<PAYABLE-FOR-SECURITIES> 0
<SENIOR-LONG-TERM-DEBT> 0
<OTHER-ITEMS-LIABILITIES> 34,279
<TOTAL-LIABILITIES> 34,279
<SENIOR-EQUITY> 0
<PAID-IN-CAPITAL-COMMON> 0
<SHARES-COMMON-STOCK> 42,939
<SHARES-COMMON-PRIOR> 45,303
<ACCUMULATED-NII-CURRENT> 58,169
<OVERDISTRIBUTION-NII> 0
<ACCUMULATED-NET-GAINS> -3,252
<OVERDISTRIBUTION-GAINS> 0
<ACCUM-APPREC-OR-DEPREC> -52,070
<NET-ASSETS> 4,185,319
<DIVIDEND-INCOME> 0
<INTEREST-INCOME> 233,052
<OTHER-INCOME> 0
<EXPENSES-NET> 6,622
<NET-INVESTMENT-INCOME> 226,430
<REALIZED-GAINS-CURRENT> -4,992
<APPREC-INCREASE-CURRENT> -126,998
<NET-CHANGE-FROM-OPS> 94,440
<EQUALIZATION> 0
<DISTRIBUTIONS-OF-INCOME> 229,399
<DISTRIBUTIONS-OF-GAINS> 0
<DISTRIBUTIONS-OTHER> 0
<NUMBER-OF-SHARES-SOLD> 50,000
<NUMBER-OF-SHARES-REDEEMED> 7,061
<SHARES-REINVESTED> 0
<NET-CHANGE-IN-ASSETS> -361,486
<ACCUMULATED-NII-PRIOR> 61,137
<ACCUMULATED-GAINS-PRIOR> 1,740
<OVERDISTRIB-NII-PRIOR> 0
<OVERDIST-NET-GAINS-PRIOR> 0
<GROSS-ADVISORY-FEES> 0
<INTEREST-EXPENSE> 0
<GROSS-EXPENSE> 0
<AVERAGE-NET-ASSETS> 0
<PER-SHARE-NAV-BEGIN> 0
<PER-SHARE-NII> 0
<PER-SHARE-GAIN-APPREC> 0
<PER-SHARE-DIVIDEND> 0
<PER-SHARE-DISTRIBUTIONS> 0
<RETURNS-OF-CAPITAL> 0
<PER-SHARE-NAV-END> 0
<EXPENSE-RATIO> 0
[AVG-DEBT-OUTSTANDING] 0
[AVG-DEBT-PER-SHARE] 0
</TABLE>
