A 66-year-old Japanese man with systemic sclerosis had benign monoclonal gammopathy. Penicillamine therapy resulted in a hypersensitivity state, with marked eosinophilia. Later he developed polymyositis. Remission of the polymyositis and a decrease in the monoclonal immunoglobulin followed prednisolone therapy. Immunological disorders may play an important role in the pathogenesis of polymyositis.