We report a case of IDDM which occurred during interferon therapy for chronic hepatitis. A 31-year-old man intermittently received 2.5 x 10(8) units of alpha-IFN and 1 x 10(8) units of beta-IFN for treatment of chronic viral hepatitis type B. Four years after the beginning of IFN therapy, he acutely developed moderate hyperglycemia and severe ketonuria with positive islet cell antibody, and then 28 units/day of insulin injection was started. After the start of insulin therapy, there was a remission period for about 3 years but insulin-dependency recurred thereafter. The clinical course of this case indicates that IFN therapy precedes IDDM. During and after IFN therapy we should consider the possibility of occurrence of IDDM as well as other autoimmune diseases and observe the clinical course carefully.