To determine the effects of interferon treatment, we studied 77 Japanese patients with hepatitis C virus (HCV) infection and normal alanine aminotransferase (ALT). Of 77 patients, 37 were given natural interferon-alpha for 24 weeks, and 40 not given interferon acted as controls. Serum samples were tested for HCV RNA and genotypes by polymerase chain reaction (PCR). HCV RNA levels were measured by competitive PCR. Of 37 treated patients, 11 (29.7%) had sustained elimination throughout a six-month follow-up, while HCV RNA was not eliminated in any untreated patients. At 24 months, the number of patients with elevated ALT was not significantly different between treated (13.5%) and untreated patients (15%). Interferon eliminates HCV RNA in patients with normal ALT without severe side effects. The natural history of HCV infection should be clarified so that the interferon treatment regimen can be tailored to the needs of each patient.