In a prospective study, 179 adult women (age range, 17-82 years) were followed up for 12 months after an index episode of community-acquired cystitis caused by Escherichia coli. Episodes of symptomatic urinary tract infection (UTI) were recorded, and urinary isolates were compared with the index episode isolate; 147 UTI episodes were detected during the follow-up. Of these episodes, 131 were classified as recurrences occurring at least 1 month after the index episode; 44% of the patients had recurrences. A history of UTI increased the risk of recurrence; only 11.8% of the 17 patients without previous episodes of UTI had at least one recurrence, while 47.5% of those with previous episodes had at least one recurrence (OR, 6.8; univariate logistic regression). E. coli caused 78% of the recurrent episodes. Phenotypic and genotypic analysis of E. coli strains showed that one-third of the recurrences were caused by the index episode strain, which could persist and cause recurrences throughout the 1-year follow-up period. The prevalence of adhesins or other identified virulence factors for UTI among the recurrence strains was identical to that among the index episode strains. The presence of these factors did not affect the risk of recurrence but did increase the likelihood that the index episode strain would persist and cause recurrent episodes of UTI.