In 14 patients with polymyositis (PM), 5 patients (2 males and 3 females) were positive for anti-hepatitis C virus (HCV) antibody measured by a second generation assay. We analysed the clinical characteristics and histopathological findings of the biopsied muscles from those 5 patients. They aged from 42 to 65 years averaging 53.6 years. Two asymptomatic patients visited our hospital due to elevated muscle enzyme levels, who had slight weakness in their orbicularis oculi and neck muscles on physical examination. The other 3 patients had moderate weakness of the proximal muscles. Anti-nuclear antibody was positive in 2 of the 5 patients and anti-Jo 1 antibody was negative in all patients. The serum enzymes elevated were creatine kinase (215-2, 207 (IU/l)) and glutamate oxaloacetate transaminase (40-119 (KU)). HCV-RNA was positive in the sera of 4 patients examined. All muscle biopsy specimens revealed variation in fiber size with inflammatory cellular infiltration and observed degenerating and regenerating fibers. The scant infiltration type was observed in 2 asymptomatic patients in whom the infiltrated cells were CD4 positive. The endomysial infiltration type was observed in 3 symptomatic patients; CD8 positive cells were found focally to diffusely in 2 patients examined. The expression of class 1 molecules from the major histocompatibility complex was detected mainly in infiltrated fibers to variable degrees. All of the patients showed a good response to the initial steroid therapy. The present study suggests that autoimmune reaction related to HCV infection causes myositis, therefore anti-HCV antibody should be checked in cases of PM.