Patients with urinary tract infection were treated for 8 to 15 days with one daily intramuscular injection of 160 mg of gentamicin or 60 or 80 mg every 8 h. Ten of 11 patients treated with one injection daily were cured as compared with 8 of 10 patients treated with three injections daily. Urinary concentrations of gentamicin were 3.2 to 600 mug/ml in all 8-h collections in patients receiving one injection daily and were 22 to 440 mug/ml in all 8-h collections in patients receiving three injections daily. The group treated once daily demonstrated a significant but reversible increase in mean serum creatinine concentration and decrease in mean creatinine clearance during therapy as compared with no change in the group treated with three injections daily. Decreases in renal function were correlated with higher milligram per kilogram doses and higher 1-h serum concentrations.