An 8-year-old girl with partial ornithine carbamoyl transferase deficiency was treated with sodium benzoate (200 mg/kg/day) for 13 months. Before administration of sodium benzoate, her protein intake was reduced to 1.0 to 1.5 g/kg/day and her caloric intake fluctuated. Hyperammonemic attacks were frequently observed in winter. After the start of administration of sodium benzoate, the severity and frequency of these attacks decreased, although her protein intake was increased to 1.5 to 2.0 g/kg/day. No adverse effect of sodium benzoate were detected by clinical and laboratory examinations. It is concluded that long-term oral administration of sodium benzoate was effective in reducing the frequency and severity of hyperammonemic attacks in this patient. Sodium benzoate therapy in combination with dietary manipulation may improve the growth and development of these patients by allowing reduced dietary protein restriction.