Thirty women were randomized in a double-blind, placebo-controlled study to receive either norfloxacin, 200 mg orally daily at bedtime, or placebo for the prevention of recurrent bladder infection. Subjects were followed monthly to monitor compliance and symptoms, for urine culture and periurethral and anal canal swabs to monitor colonization, and for blood specimens for hematologic and biochemical studies to monitor safety. During 1 year of follow-up, 10 of 15 placebo subjects and none of 15 norfloxacin subjects developed infection (P less than 0.001). Adverse effects occurred with equal frequencies in the two groups. For norfloxacin subjects, only 2 (1.6%) of 129 periurethral and 4 (3.1%) of 129 anal canal swabs showed colonization with aerobic gram-negative organisms, while 16 (22%) of 73 periurethral and 47 (64%) of 73 anal canal swabs from placebo subjects showed colonization. Daily therapy with norfloxacin at bedtime is effective in preventing recurrent cystitis. During 1 year of norfloxacin therapy, colonization was infrequent and superinfection with norfloxacin-resistant organisms did not occur.