Interferons (IFNs) have immunomodulatory properties such as direct increase in the production of pathogen autoantibodies, enhanced cytotoxic T cell and B cell activities, inhibition of T suppressor cell function and induction of HLA class I antigen expression. These actions of IFN induce autoimmune disorders including autoimmune thyroiditis, hemolytic anemia and thrombocytopenia, SLE, rheumatoid arthritis and psoriasis, however, these autoimmune diseases except for autoimmune thyroiditis, are rare among side effects of IFN therapy. Most of the patients showing these autoimmune disorders during IFN treatment have predisposal immunological abnormalities such as positive ANA and antithyroidal autoantibodies. We described here autoimmune disorders during and after IFN treatment among 1023 cases with chronic active type C hepatitis. The cases with SLE, thrombocytopenic purpura, rheumatoid arthritis and psoriasis showed good prognosis after cessation of IFN administration.