This prospective, open, randomized clinical study compared the efficacy and tolerability of norfloxacin and ciprofloxacin in adult patients with complicated urinary tract infection, defined as infection in the presence of an underlying anatomic or functional abnormality of the urinary tract. Seventy-two patients were randomized, 37 received norfloxacin (400 mg orally twice daily for 10-21 days) and 35 received ciprofloxacin (500 mg orally every 12 hours for 14-21 days). Patients were clinically assessed, and urine cultures were obtained following two to four days of therapy, and five to nine days and four to six weeks after discontinuation of therapy. Seventy-two percent of the norfloxacin group and 79 percent of the ciprofloxacin group were considered cured. This difference was not significant. One failure of norfloxacin therapy was associated with the emergence of resistant Pseudomonas aeruginosa. Following norfloxacin therapy, in 2 patients superinfections developed with resistant organisms (Staphylococcus epidermidis [1] and Pseudomonas maltophilia [1]). Twelve patients, 6 in each group, experienced adverse reactions, which were considered related to the study drug therapy in only four instances. Our results indicated comparable efficacy and tolerability of norfloxacin and ciprofloxacin in the treatment of complicated urinary tract infection in adults.