Antimicrobial prophylaxis prevents recurrent urinary tract infections in susceptible women, but its cost effectiveness has not been studied. In a recent placebo-controlled trial of urinary prophylaxis, we also assessed cost effectiveness using a decision analysis model. In our hospital the direct cost of 1 patient year of urinary prophylaxis approximates the cost of treating one episode of cystitis. In women with a baseline infection rate of three per patient year, the annual cost of prophylaxis ($85.82) was less than treatment of acute episodes of infection ($392.30). Sensitivity analyses showed that in women with three infections per year, prophylaxis became cost effective when charges per episode exceeded $42.00. In women with frequent episodes of cystitis, prophylaxis will be cost effective in most practice settings.