To determine the efficacy and safety of single-dose oral ciprofloxacin prophylaxis for the prevention of post-operative bacteriuria following transurethral resection of the prostate or bladder tumour, a prospective, randomized, double-blind, placebo-controlled trial was conducted. Five hundred and eighteen patients were randomized in a 2:2:1 ratio to receive ciprofloxacin 500 mg, cefotaxime 1 g or placebo 30-90 min before surgery. Of the 368 efficacy-evaluable patients, five (3.3%) ciprofloxacin, seven (4.8%) cefotaxime and five (7.0%) placebo recipients had post-operative bacteriuria (> or = 10(4) cfu/mL) during post-operative days 2-15. Five (3.4%) ciprofloxacin, five (3.4%) cefotaxime and one (2.4%) placebo recipients were considered clinical failures, of whom one, two and one patients, respectively, had concomitant bacteriuria. Drug-related adverse events were reported in six of 204 (3%) ciprofloxacin, 12 of 197 (6%) cefotaxime and one of 101 (1%) placebo patients. The observed rates of post-operative bacteriuria suggest that a single 500 mg dose of ciprofloxacin is suitable prophylaxis for transurethral surgery.