Fourteen patients with chronic or multiple recurrences of infection of the urinary tract have self-administered a single oral dose of one of five antibiotics after sexual intercourse for periods of 19 to 111 months for a total of 761 months. Infections did not occur among 15 of 22 treatment periods. A total of 19 infections occurred while the patients were taking prophylactic medication, significantly less than the total of 90 infections recorded during the 705 months when these patients did not take prophylactic doses of antibiotics. Patients taking nitrofurantoin, a cephalosporin, or nalidixic acid had a significant reduction in the proportion of specimens of urine containing any Gram-negative bacteria. Serious toxic effects were not encountered.