A total of 21 sexually active premenopausal women, prone to recurrent urinary tract infections but who otherwise were healthy, underwent post-coital prophylaxis consisting of bladder voiding and a single 250 mg. tablet of cinoxacin. While 94 urinary tract infections occurred during a mean of 7.5 months before treatment, only 8 occurred during a mean of 12.5 months after prophylaxis. This difference was statistically highly significant. A mean of 106 cinoxacin tablets per patient were administered during post-coital prophylaxis. Cinoxacin represents an additional valuable and effective antibacterial in post-coital prophylaxis of recurrent urinary tract infection in otherwise healthy premenopausal women, although it is slightly less effective than cotrimoxazole or nitrofurantoin. Effective post-coital prophylaxis requires the use of much smaller quantities of antibacterial agents than the daily use of a single tablet and in women with a high incidence of recurrent urinary tract infections it is superior to intermittent self-administered antibacterial therapy.