We evaluated the toxicity and efficacy of netilmicin and tobramycin in 89 older adults with serious bacterial infections and pre-existing renal impairment in a prospective, blinded, randomized trial. Complete resolution or improvement of infection occurred at 34/36 (94%) evaluable sites in netilmicin-treated patients and at 26/31 (84%) evaluable sites in tobramycin-treated patients. 10/44 (23%) netilmicin- and 7/45 (16%) tobramycin-treated patients experienced nephrotoxicity during treatment. The mean serum creatinine level improved significantly at the end of therapy compared to pre-treatment in those patients who did not experience nephrotoxicity in both treatment groups. 5/19 (26%) netilmicin-treated patients and 2/18 (11%) tobramycin-treated patients assessable for ototoxicity experienced decrements in auditory thresholds. Ototoxic netilmicin-treated patients had higher serum netilmicin levels than did non-ototoxic patients. Patients who experienced ototoxicity were not more likely to have experienced nephrotoxicity. The rates of toxicity were not statistically different and were similar to those seen in studies of patients with normal pre-treatment renal function.