To assess the significance of sexual intercourse as a precipitating factor of urinary tract infection in premenopausal women with recurrent infection, 15 patients were monitored with daily dipslides and calendar recording of intercourse episodes after antimicrobial prophylaxis was discontinued. Eleven patients experienced 16 infections; 12 infections occurred within 24 hr of an intercourse episode, two occurred during the menstrual period, and two had no recognizable association. In 12 control subjects, followed in a similar fashion, three infections--all within 24 hr of an intercourse episode--were documented. There was no difference in frequency of intercourse between patients and controls, but the frequency was higher in infected women than in uninfected women (P less than 0.02). These data suggest that in sexually active women, most urinary tract infections are intercourse-related.