Three hundred seventy patients with upper or lower urinary tract infections were entered into a multicenter, open comparative study. A total of 190 patients were treated with norfloxacin, and 180 patients were treated with trimethoprim-sulfamethoxazole. The percentage of strains susceptible to norfloxacin (99%) was significantly greater (P less than 0.001) than the percentage of strains susceptible to trimethoprim-sulfamethoxazole (90%). The percentages of patients with bacteriological outcomes of eradication were greater in the norfloxacin group (97%) than in the trimethoprim-sulfamethoxazole group (90%). The difference was significant (P less than 0.05). Seven patients (three treated with norfloxacin, four treated with trimethoprim-sulfamethoxazole) experienced early reinfection. Of 370 patients entered into the study, 20 patients experienced clinical adverse effects that were probably or definitely related to the study drug; 6 patients were in the group that received norfloxacin, and 14 were in the group that received trimethoprim-sulfamethoxazole. Study antimicrobial agents were discontinued because of clinical adverse effects in eight patients (norfloxacin, one patient; trimethoprim-sulfamethoxazole, seven patients). Three patients receiving norfloxacin and four patients receiving trimethoprim-sulfamethoxazole had laboratory adverse effects which were classified as probably or definitely drug related. None of the clinical or laboratory adverse effects was serious.