Polymyositis: An autoimmune inflammatory disease of muscle that begins when white blood cells spontaneously invade muscles, especially those closest to the trunk or torso. This immune activity results in muscle pain, tenderness, and weakness. Blood testing in a person with polymyositis shows significantly elevated creatinine phosphokinase levels. The diagnosis is further suggested by electromyogram testing and confirmed with muscle biopsy. Treatment of polymyositis requires high doses of cortisone-related medications, such as prednisone, and immune suppression with medications, such as methotrexate and cyclophosphamide.