We describe a case of polymyositis (PM) with liver injury that occurred in a patient with rheumatoid arthritis (RA). A 74-year-old woman who had a 12-year history of RA was admitted to our hospital because of muscle weakness and liver dysfunction. CD8-positive T cell infiltration was found in the interstitium of both the liver and muscle. In addition to the administration of a large amount of prednisolone (PSL), high-dose intravenous immunoglobulin (IVIG) successfully improved myositis and hepatitis. Our case indicates the pathogenic potential of CD8-positive T cells in PM-associated liver injury.