We studied 56 sexually active premenopausal women with a normal genitourinary tract but with persistent introital colonization by enteric gram-negative bacteria who were prone to suffer recurrent urinary tract infections. Sexual intercourse was revealed as a major factor in inducing recurrent urinary tract infections, usually within 24 hours, by transferring the pre-existing introital bacteria into the bladder. Abstention from sexual activity without any additional treatment prevented the development of new urinary tract infections in these women despite persistent introital enteric bacterial flora. Urinary tract infections occurred after onset of sexual activity in all but 2 of the premenopausal women in this study. The 25 premenopausal women with recurrent urinary tract infections were subjected to early postcoital prophylaxis consisting of bladder voiding and the administration of a single tablet of either cotrimoxazole (80 mg. trimethoprim plus 400 mg. sulfamethoxazole), nalidixic acid (500 mg.), nitrofurantoin (50 or 100 mg.) or sulfonamides (500 mg. sulfisoxazole or 250 mg. sulfamethizole). Whereas 70 urinary tract infections occurred during a mean 8-month followup before treatment only 4 occurred during the mean 12.5-month followup after introduction of post-coital prophylaxis (none occurred on co-trimoxazole or nalidixic acid therapy and only 1 infection occurred on nitrofurantoin therapy). Sulfonamides are not recommended as post-coital prophylaxis because of the higher incidence of breakthrough infections. Post-coital prophylaxis with co-trimoxazole, nalidixic acid or nitrofurantoin proved to be simple, economical and efficient, and is recommended in the prevention of recurrent urinary tract infections in otherwise normal premenopausal women.