We studied pharmacokinetic variables in 100 neonates to evaluate the need for a loading dose of gentamicin. The mean volume of distribution for gentamicin in this population was .542 \pm .205 L/kg. Forty-five percent of these patients would not achieve peak serum concentrations of 5 mUg/ml after one dose of 2.5 mg/kg. A loading dose of 4 mg/kg would result in a level of 5 mUg/ml in 92% of the patients. After this study, a 4 mg/kg loading dose was initiated at our institution. Peak and trough concentrations at steady state were compared in 50 patients receiving a loading dose and 40 patients who had not received a loading dose. No significant differences were found in peak or trough concentrations after three or more doses, verifying that a loading dose does not affect steady-state concentrations. Because gentamicin toxicities are related to accumulation over time, whereas therapeutic efficacy may be related to early attainment of therapeutic peak serum concentrations, we recommend a loading dose of 4 mg/kg in all neonates beginning gentamicin therapy.