In a double-blind multicentre study the efficacy and safety of a single-dose treatment with pefloxacin (800 mg) was compared with a five-day treatment regimen of 960 mg co-trimoxazole twice daily in the therapy of acute uncomplicated cystitis in women. In order to maintain blindness, patients in the pefloxacin group received placebo to complement the full number of tablets. Nine centres were involved; 155 patients received pefloxacin and 161 patients received co-trimoxazole. Of these, 140 patients treated with pefloxacin and 145 with co-trimoxazole were considered valid for efficacy and safety analysis. At the first follow-up, after seven to ten days, 97.1% of the pefloxacin group and 95.2% of the co-trimoxazole group were bacteriologically cured. At the second follow-up visit, after 28 to 42 days, the urine culture was negative in 95.0% of the pefloxacin group and 90.3% of the co-trimoxazole group. A single dose of 800 mg pefloxacin was demonstrated to be as safe and at least as effective as a five-day regimen of co-trimoxazole in the treatment of uncomplicated cystitis.