From January 2001 to July 2002, we investigated the duration of fever, the duration of hospitalization, the frequency of antipyretic use, and other clinical symptoms of 162 inpatients with influenza A virus infection, and compared them with oseltamivir-treated, amantadine-treated, and untreated groups. The duration of fever and the duration of hospitalization treated were significantly shortened in the oseltamivir-treated group than in the amantadine-treated group and untreated group. There was no difference in the duration of fever between patients treated by oseltamivir at 2 mg/kg/day and those at 4 mg/kg/day. The frequency of antipyretic use was lower in the oseltamivir-treated group than in the other group. No difference was observed in the duration of fever and the frequency of antipyretic use between patients treated by oseltamivir with antibiotics and those by oseltamivir alone. The complications such as vomiting, abdominal pain, irritability were observed in 9% of patients treated by oseltamivir. But those symptoms were not serious, and the rate of complications in the oseltamivir-treated group was lower than that in untreated group. In conclusion, oseltamivir is safe and effective in the treatment of influenza virus infection in children, and it may reduce the amount of antibiotics and antipyretic use.