It is well known that mood disorder such as depression occasionally develops during interferon (IFN) therapy for chronic viral hepatitis. So far, however, IFN-induced manic disorder has been rarely reported. We present a case of manic psychosis which developed during IFN treatment for chronic hepatitis C. A 35-year-old man with chronic hepatitis positive for hepatitis C virus RNA in serum was treated with natural IFN alpha with a daily dosage of 5 million units. Six weeks later he complained of insomnia, and then became exhilarated, talkative, restless and aggressive. Since the mental state was compatible with manic disorder, IFN therapy was immediately ceased. Simultaneously, psychotropic drugs were administered. One week later, the psychiatric disturbances disappeared. He has been keeping his usual social interactions without the psychotropic drugs after that. It is suggested that manic psychosis happened secondary to IFN alpha treatment.