We report a case of alanine aminotransferase (ALT) deficiency in a 68-year-old Japanese female with chronic hepatitis C. The serum was positive for antibody to hepatitis C virus (HCV) and HCV-RNA. Liver biopsy showed histological evidence of chronic active hepatitis. The level of serum aspartate aminotransferase (sAST) was elevated, but sALT was extremely low. The patient was followed up for her serum aminotransferase levels for 1.5 years under the treatment with ursodeoxycholic acid. The low sALT level persisted during all the follow-up period. The ALT activity in liver tissue was also decreased. Based on these findings, ALT deficiency was suspected. sALT activity was also found to be low in her two sons. This latter finding suggests the hereditary character of this abnormality.