We report a case of chronic hepatitis C in whom liver cirrhosis was later diagnosed following abnormality of ALT levels during pegylated interferon α2a and ribavirin treatment. A 62-year-old woman with chronic hepatitis C was treated with pegylated interferon α2a plus ribavirin for 72 weeks. Her HCV RNA became negative 16 weeks after the start of treatment and continued to be negative for most of the treatment duration. Her AST/ALT, ALP/γ-GTP levels became elevated soon after the initiation of treatment and thereafter remained unchanged. However, most of these levels normalized after the end of treatment. Post-treatment liver biopsy showed liver cirrhosis, probably due to the interferon treatment itself. This unusual therapeutic outcome should be considered if the levels of hepatic dysfunction during interferon treatment are severe.