Polymyositis is non-hereditary, autoimmune rheumatic disease characterized by chronic inflammation of skeletal muscles. The disease showed relatively good response to treatment compared to the other myopathy. There are a few reports of the clinical course of patients with polymyositis and an analysis of their responses to therapy, especially at the Department of Rehabilitation Medicine in Korea. So we report three cases of polymyositis treated with the steroid. This report emphasizes that the serum enzymes, creatine phosphokinase and lactic dehydrogenase levels were more sensitive than muscle strength tests or clinical symptoms in recording the response to treatment in patients.