The efficacy and safety of 7-10-day courses of lomefloxacin (single daily dose of 400 mg) or norfloxacin (twice-daily doses of 400 mg) for the treatment of uncomplicated urinary tract infections were compared in two large, multicenter, randomized trials. This article presents the combined results of these trials, which were conducted in a total of 27 centers throughout the United States. A total of 727 adults, mostly women, with symptoms of acute urinary tract infection were enrolled; 370 patients were randomized to lomefloxacin treatment, and 357 received norfloxacin. The bacteriologic cure rate at 5-9 days post-therapy was 98.2% in the lomefloxacin group and 96.3% in the norfloxacin group (p = nonsignificant). The clinical success rate of 99.1% in the lomefloxacin group was significantly higher than the success rate of 93.5% in the norfloxacin group (p = 0.002). Adverse events were reported by 157 lomefloxacin-treated patients and 129 patients receiving norfloxacin. Adverse events attributable to drug treatment occurred in 41 patients (11.1%) in the lomefloxacin group and 27 (7.6%) in the norfloxacin group. Eight lomefloxacin (2.2%) and three norfloxacin patients (0.8%) were withdrawn from treatment because of adverse events probably attributable to the drug. The incidence of dizziness, tremor, and photosensitivity rash was higher in the lomefloxacin group than in the norfloxacin group, while the incidence of nausea was higher in the norfloxacin group. The results of these trials demonstrate that once-daily administration of 400 mg lomefloxacin is as safe and effective clinically as, and superior bacteriologically to, twice-daily administration of 400 mg norfloxacin in the treatment of acute uncomplicated urinary tract infections in adult patients.