The efficacy and tolerance of netilmicin was studied in 28 elderly male patients with varying degrees of renal function impairment who suffered from complicated urinary tract infections. Doses of netilmicin, equivalent to 2 mg/kg divided by milligrams of creatinine per 100 ml, were administered every 12 h. A 62% cure rate, defined as negative urine culture at 1-week follow-up, was obtained. Treatment failure correlated with impaired renal function. Nephrotoxic reaction, defined as any significant increase in serum creatinine during treatment, was found in 6 of 28 patients (21%). The increase in serum creatinine was transient in all except one of these patients. Apart from the finding of a significant correlation between nephrotoxic reaction to netilmicin and postoperative urinary tract infection, no clinical or therapeutic features correlated with nephrotoxicity; trough concentrations correlated with serum creatinine.