Inclusion body myositis (IBM) is a chronic progressive inflammatory myopathy in elders. Three patients with chronic hepatitis C developed IBM. Their muscle biopsies were examined by polymerase chain reaction (PCR) for two patients and immunohistochemistry for three patients. The hepatitis C virus (HCV)-RNA genome was detected in one of the two patients. The degenerated and atrophic muscle fibers were immunoreactive for an anti-HCV antibody in all three patients. Immunoreactivity for an anti-HCV antibody was not apparent in the muscle specimen from an IBM patient without HCV infection. Our findings suggest that HCV infection is related to the pathogenesis in some cases of IBM.