The nephrotoxicity of tobramycin given at a dose of 4·5 mg/kg/day for a period of 12 days to a group of 90 patients with a mean age of 62·9 years was studied. Toxicity was determined on the basis of 3 main criteria (oliguria <400 ml/24 hr, serum creatinine 0·4 mg increase over a minimum basal level of 1·2 mg/100 ml, BUN 5 mg increase over a minimum of 25 mg/100 ml); and 3 minor criteria (proteinuria, microhaematuria and cylindruria). These parameters were determined before treatment at 7, 10, 14, 17, 21, and 30 days afterwards. The age and coexistence of factors such as hypertension, diabetes, anaemia, cardiac insufficiency, shock and dehydration were considered. Nephrotoxicity level ranges from 3·3 to 38·8% depending on the criterion used, and is related to hypertension (P<0·001), age (P<0·005) and association with ampicillin (P<0·005). Nephrotoxicity was reversible spontaneously in 96·7% of the cases and no differences have been observed between patients with moderate renal insufficiency and those with normal renal function on the initiation of treatment.