A prospective comparative study was undertaken to assess the relative nephrotoxicity of tobramycin and gentamicin. Patients receiving these drugs in the medical and surgical wards of the Royal Prince Alfred Hospital, Sydney, over a six-week period were included in the study. There was 33 patients treated with gentamicin and 19 treated with tobramycin (mean age, 52.6 years and 42.9 years respectively). Nine patients (27%) treated with gentamicin and two (10%) treated with tobramycin developed abnormally elevated plasma creatinine levels (greater than 110 mumol/L). Of patients with normal plasma creatinine level at the beginning of therapy, no significant change in the plasma level of creatinine was noted in the group treated with tobramycin, whereas those treated with gentamicin showed a significant increase in plasma creatinine level with time. It was concluded that tobramycin was significantly less nephrotoxic than gentamicin when used in typical clinical setting. With both drugs, monitoring of renal function is essential during therapy, particularly if other drugs excreted by the kidney are administered.