We herein describe a case of myelodysplastic syndrome with chronic graft-versus-host disease (cGVHD)-related polymyositis. On approximately day 1,570 post HLA-identical sibling bone marrow transplant, the patient presented with a fever, myalgia and liver dysfunction. A muscle biopsy revealed destruction of the muscle fibers and infiltration of CD20 + B cells and CD4 + and CD8 + T cells, and a liver biopsy confirmed the findings of cGVHD. An analysis of plasma cytokine profiles indicated elevation of not only T-helper (Th)1 and Th2, but also Th17 cytokines. Increases in these cytokines in addition to the invasion of inflammatory cells might be associated with the pathophysiology of cGVHD involving the muscle and liver.