Chronic graft-versus-host disease (GVHD) sometimes mimics autoimmune diseases. We report the case of a 39-year-old patient who presented atypical polymyositis without elevated creatinine phosphokinase, related to a chronic GVHD following interruption of immunosuppressive treatment. Treatment with cyclosporine and corticosteroids resulted in complete and sustained remission of the polymyositis. The symptoms of chronic GVHD-related polymyositis are indistinguishable from those of idiopathic polymyositis. The context of transplantation and a decrease or interruption of prophylaxis suggest the diagnosis of GVHD-related polymyositis, especially if other manifestations of GVHD are associated. A suitably adapted treatment (association of corticotherapy and cyclosporine) improves polymyositis, and in most cases, a normal clinical state is achieved even if the symptoms were severe.