Polymyositis may occur along with other manifestations of chronic graft vs host disease after allogeneic bone marrow transplantation (BMT). Donor lymphocyte infusion (DLI) could produce durable remissions in relapsed patients with chronic myelogenous leukemia (CML) but it may contribute to the development of polymyositis. We report in this study a 25-year-old man who suffered from a relapse of CML 4 years after a sibling human leukocyte antigen-matched allogenic BMT. The patient developed polymyositis 18 months after DLI. Mini-pulse therapy with methylprednisolone was effective for his proximal weakness and elevated creatine phosphokinase. There was no relapse of symptoms of polymyositis on tapering of the medication.