Abstract
We herein report a 70-year-old man with malaise and muscle weakness that had developed within a month. The patient also had abdominal fullness due to polycystic kidney disease. Severe proximal skeletal muscle weakness and mild elevation of creatinine kinase to 301 IU/L were noted. A muscle biopsy of the right bicep showed polymyositis. Computed tomography showed a right renal mass, and an analysis after right nephrectomy identified clear cell carcinoma. The muscle weakness subsided one month after nephrectomy and intravenous immunoglobulin therapy. Therefore, we suspect that the development of polymyositis in this patient was closely related to renal cell carcinoma.