Abstract: This paper reports the ultrastructural changes of the striated muscles in 5 cases of dermatomyositis/polymyositis. We found that the degenerative myofibrils observed under light microscope lost their ultrastructures of myofilaments, mitochondria and sarcoplasmic reticula. The A bands and I bands could not be distinguished. The necrotic myofibrils also lost their ultrastructures of Z lines and sarcomeres besides the abovementioned Changes. The characteristic structures of myofibrils could not be observed. Some abnormal ultra structures could be found in myofibrils with normal appearance under light microscope.