The efficacy and safety of intramuscular administration of either of the two aminoglycoside antibiotics tobramycin and gentamicin every 8 hr for seven days were compared in a prospective, randomized study of 75 elderly male patients with complicated urinary tract infections. All bacteria were sensitive to both antibiotics by the disk diffusion method. The two groups of patients (each receiving one of the two drugs) were comparable in terms of infecting microorganisms and underlying pathology of the urinary tract (mostly prostatic hyperplasia or carcinoma and urethral strictures). No patients had indwelling catheters. All patients had normal renal function (serum creatinine, less than or equal to 1.5 mg/100 ml and/or blood urea nitrogen, less than or equal to 25 mg/100 ml). In both groups 64% of the patients were cured of the infection, with cure defined as a negative urine culture result seven days after discontinuation of therapy. A statistically but not clinically significant increase in the level of serum creatinine in both groups of patients was noted after treatment.