Polymyositis and myasthenia gravis-like syndromes have been seen in patients with GVH disease following bone marrow transplantation. We therefore investigated the histopathology of muscle in mice with acute graft-versus-host disease in order to determine whether these conditions are caused by injury from the GVH reaction itself or are due to radiation and drugs used to prepare the host for transplantation. GVH reactions were induced by intravenously infusing 50×506lymph node and spleen cells from A/J-strain donors into (C57BL/6×A/J)F1-hybrid recipients. These mice developed an active inflammatory myopathy beginning 15 days after engraftment. The inflammatory infiltrates were focal in distribution, initially around perimysial blood vessels, and later around muscle fibers. The infiltrating cell population was composed of lymphocytes, plasmacytoid cells, and macrophages. Muscle cell necrosis was observed and was temporally related to elevations in serum creatine kinase. Similar histologic changes were present in the myocardium. Our findings support the notion that muscle involvement in patients with GVH disease is caused by the disease itself. Myositis accompanying experimental GVH disease in mice may hold promise as a model of autoimmune inflammatory myopathy.