Background. Approximately 30%of patients with chronic hepatitis C have normal serumalanine amino transferase (ALT) levels. While interferon (IFN) mono therapy is approved for patients with chronic hepatitis C infection, the effectiveness of such therapy for chronic hepatitis C patients with normal ALT levels at commencement of treatment remains poorly understood. Methods. Ninety four individuals (M/F, 54∶40; median age, 46 years) with normal ALT levels (< 50 IU/ 1) at the commencement of treatment who were positive for both antihepatitis C virus (HCV) and serum HCV RNA were studied. Among this group, 18 individuals (M/F, 9∶9; median age, 50 years) had had persistently normal ALT levels for at least 3 months prior to treatment. All patients received their first course of IFN therapy in this study. Results. Forty three (45.7 %) of 94 individuals had lost serum HCV RNA at 6 months after cessation of therapy (complete response; CR). The proportion of patients with genotype 2a and HCV RNA level over 1 Meq/ml who showed CR was significantly lower in those with normal ALT levels than in those with elevated ALT levels (23.8%vs 55.6%; P = 0.0189). Two patients who had persistently normal ALT levels and HCV RNA level over 1 Meq/ml were non responders (NR) and had ALT flare ups after IFN therapy. Patients with HCV RNA levels of less than 1 Meq/ml did not show differential responses based on ALT levels. Conclusions. Our data suggest that IFN therapy is effective for patients with normal ALT levels and less than 1 Meq/ml HCV RNA. Thus, such patients should be considered for curative IFN therapy. Background: Approximately 30% of patients with chronic hepatitis C have normal serumalanine amino transferase (ALT) levels. While interferon (IFN) mono therapy is approved for patients with chronic hepatitis C infection, the effectiveness of such therapy for chronic hepatitis C patients with normal ALT levels at commencement of treatment remains poorly understood. Methods. Ninety four individuals (M / F, 54:40; median age, 46 years) with normal ALT levels (<50 IU / 1) at the commencement of treatment who were positive for All this group, 18 individuals (M / F, 9: 9; median age, 50 years) had had persistently normal ALT levels for at least 3 months prior to treatment. All patients received their first course of IFN therapy in this study. Results. Forty three (45.7%) of 94 individuals had lost serum HCV RNA at 6 months after cessation of therapy (CR). The proportion of patients with genotype 2a and HCV R NA levels over 1 Meq / ml who showed CR was significantly lower in those with normal ALT levels than those with elevated ALT levels (23.8% vs 55.6%; P = 0.0189). Two patients who had persistently normal ALT levels and HCV RNA levels over 1 Meq / ml were non responders (NR) and had ALT flare ups after IFN therapy. Patients with HCV RNA levels of less than 1 Meq / ml did not show differential responses based on ALT levels. Conclusions. Our data suggest that IFN therapy is effective for patients with normal ALT levels and less than 1 Meq / ml HCV RNA.