Netilmicin and amikacin, two recently developed aminoglycosides, were compared in a prospective, randomized study of 57 male patients with complicated urinary tract infections. Both drugs were administered intramuscularly every 12 h for 7 to 10 days, netilmicin at 2 mg/kg and amikacin at 7.5 mg/kg. The two groups were comparable as to infecting bacteria and underlying pathology of the urinary tract. No patients had indwelling catheters. All microorganisms isolated were sensitive to both antibiotics. A total of 69% of the patients treated with netilmicin and 57% of the patients treated with amikacin were cured of the infection, as defined by a negative culture at 7 days after discontinuation of treatment. No major side effects were recorded, and no significant changes were noted in parameters of renal function. Of the patients treated with amikacin, 21% experienced temporary local pain at the injection site; no such effect was noted in the netilmicin-treated group. Therefore, netilmicin appeared to be as effective and better tolerated than did amikacin in the treatment of complicated urinary tract infection.