Forty patients with urinary tract infections were randomly assigned to receive a ten-day course of oral therapy with either norfloxacin 400 mg twice daily or cotrimoxazole (trimethoprim-sulfamethoxazole) 160/800 mg twice daily. There were 34 cases (19 in the norfloxacin and 15 in the cotrimoxazole group) of evaluable infections due to Escherichia coli (85% of cases), Klebsiella pneumoniae, Enterobacter spp., Proteus vulgaris and Alcaligenes faecalis. All organisms were sensitive to the assigned study drug. Twenty-two strains of Escherichia coli and five other isolates had a norfloxacin MIC50 of 0.03 mg/l and MIC90 of 1.0 mg/l. All patients were cured of the initial infection. Three diabetic patients in the norfloxacin group and another healthy patient in the cotrimoxazole group experienced asymptomatic recurrences due to organisms of the same species which, in the absence of causes of bacterial persistence, were considered to be reinfections. Mild reversible adverse effects of no clinical significance were observed in nine patients in each treatment group. Norfloxacin seems to be as effective and safe as cotrimoxazole in the conventional treatment of uncomplicated urinary tract infection.