Damage of skeletal muscle in association with graft-versus-host disease (GvHD) has been referenced exceptionally. Eighteen months after bone marrow transplantation, a 22-year-old man developed polymyositis associated with manifestations of chronic GvHD, such as peripheral eosinophilia and localized morphea. Diagnosis of polymyositis was established by clinical, electromyographic, and histopathologic findings. His clinical condition improved with immunosuppressive therapy. At electronmicroscopy, some close and broad contacts between lymphocytes with activated appearance and degenerated muscle fibers were observed, suggesting a lymphocytotoxic mechanism. The findings support the idea that polymyositis can be considered a manifestation of chronic GvHD.