BACKGROUND: Once daily dose of aminoglycoside has been used recently in the gram-negative infection for the purpose of improving efficacy. The clinical efficacy and side effects of once daily versus divided doses of gentamicin were compared in acute pyelonephritis. METHOD: Gentamicin (3-5mg/kg/day) was administered into 3 divided doses intravenously in 15 patients of the divided dose group, and the same dose was administered at a time in 19 patients of the once daily dose group. The duration of treatment was 6-14 days. RESULTS: The clinical outcome of all patients was favorable, and nephrotoxicity or ototoxicity was not detected in any patients. E. coli were isolated from 12 patients in the divided dose group, and 15 patients in the once daily dose group. They were all eradicated after treatment. The mean peak serum concentrations of gentamicin were 5.33+/-1.99;g/mL in the divided dose group, and 14.79+/-5.71g/mL in the once daily dose group. The trough concentrations were not different significantly between two groups(0.69+/-0.58;g/mL in the divided dose group vs. 0.35+/-0.45g/mL in the once daily dose group). The number of patients with peak concentration over 5.0g/mL was 8 out of 15 in the divided dose group. CONCLUSION: The once daily dose of gentamicin was as effective as the divided dose, and the nephrotoxicity or ototoxicity was not observed in both groups.