• A child, three years after bone marrow transplantation for aplastic anemia, had polymyositis develop as part of chronic graft vs host disease (GVHD). Previous reports have alluded to nonspecific musculoskeletal complaints during life and the occasional finding of necrotizing myositis at autopsy. This report describes the recognition during life, biopsy findings, and response to treatment of inflammatory myopathy during the course of GVHD.