We report a 60-year-old man with myelodysplastic syndrome (MDS) who developed polymyositis and cutaneous vasculitis. He noticed difficulty in climbing up stairs 4 months before admission. On admission, he showed brownish skin pigmentation in the distal positions of the four extremities. Neurological examination revealed muscle weakness of the neck flexor and proximal muscles of four extremities. Serum myogenic enzymes including creatine kinase increased, and electromyography showed denervation potentials and MUPs were myogenic, which was compatible with polymyositis. Muscle biopsy indicated perivascular mononuclear cell infiltration and muscle fiber necrosis with cytoplasmic bodies and rimmed vacuoles in the muscle fibers. Complete blood cell counts revealed macrocytic normochromic anemia and bone marrow puncture disclosed marked hyperplasia of megakaryocytes, erythroblasts and myeloblasts, suggesting a refractory anemia type of MDS. In addition, skin biopsy showed chronic cutaneous vasculitis. Serum protein electrophoresis and immunoelectrophoresis revealed the presence of IgM gamma type monoclonal gammopathy. With immunosuppressive therapy, serum creatine kinase level was decreased. Since immunological abnormalities and the presence of autoimmune diseases have been reported in patients with MDS, both polymyositis and chronic cutaneous vasculitis in the present patient may be caused by the similar autoimmune mechanisms related to MDS.