One hundred elderly hospitalised patients, aged 70 to 97 years (mean 81.7 years; SD 7.1 years) with a urinary tract infection were entered into a randomised study to compare the efficacy, safety and tolerance of norfloxacin 400 mg twice daily for three days with trimethoprim 300 mg once daily for three days. Forty-two of 49 patients (86%) were cured with norfloxacin, compared with 35 of 51 (69%) with trimethoprim (p less than 0.05). No patient reported any side effects during treatment. Two patients treated with norfloxacin and three treated with trimethoprim developed a disturbance of liver function. Three deaths occurred within 28 days of treatment with trimethoprim but were unrelated to the treatment. In this study norfloxacin proved superior to trimethoprim for the treatment of uncomplicated urinary tract infections in elderly hospitalised patients.