Effects of a single episode of intercourse on vaginal flora and epithelium were examined in subjects randomly assigned to groups that used no condom or lubricated nonspermicide condoms. Subjects were evaluated at visits before (1 month and 1-2 days) and after (8-12 h, 2-3 days, and 6-8 days) an index episode of sexual intercourse. The 22 subjects who used no condoms had significantly more Escherichia coli and a high concentration (> or =10(5) cfu/mL) of E. coli in the vagina (both, P<.001) and urine (all <10(5) cfu/mL; P=.004) at visit 3 than at visits 1 and 2. The 20 subjects who used condoms had a trend toward more vaginal E. coli (P=.06) and a significant increase in other enteric gram-negative rods (P=.001) after intercourse. Intercourse was not associated with gross, colposcopic, or histologic vaginal epithelial abnormalities.