Polymyositis is a rare complication of interferon alpha treatment as a result of immunemodulating role of the drug itself. In this case, interferon alpha induced polymyositis and cardiomyopathy is diagnosed in a 33-yr-old male patient with history of chronic hepatitis B. To treat hepatitis B, interferon alpha was administered until the proximal muscle weakness developed. Thereafter, sixteen cycles of immunoglobulin treatment (400 mg/kg) along with corticosteroids were instituted and led to an improvement in subjective symptoms with decreases in level of CPK and LDH. However, dilated cardiomyopathy has not improved in spite of the cessation of interferon treatment. Unlike the persistently elevated serum HBV DNA level, the serum ALT and AST levels have gradually decreased. Our case shows that clinical symptoms of polymyositis improved with steroid and immunoglobulin treatment without deterioration of the hepatitis B. To our knowledge, this is the first case of polymyositis associated with dilated cardiomyopathy after the administration of interferon in a patient with hepatitis B.