To examine whether antecedent antimicrobial use influenced subsequent relative risk of urinary tract infection (UTI) in premenopausal women, data were analyzed from two cohorts of women observed prospectively for 6 months to determine risk factors for UTI. Using a Cox proportional hazards model to adjust for covariates, we found that 326 women in a University cohort and 425 women in a health-maintenance organization cohort were at increased risks for UTI (2.57 [95% confidence interval (CI), 1.24–5.32] and 5.83 [95% CI, 3.17–10.70], respectively) if antimicrobials had been taken during the previous 15–28 days but not during the previous 3, 7, or 14 days. The increased risks were noted both for women whose antimicrobial use was for treatment of a previous UTI and for women who received antimicrobials for other illnesses. These results suggest that recent antimicrobial use increases a woman's risk of UTI, perhaps by altering the indigenous urogenital flora and predisposing to vaginal colonization with uropathogens.