Ten patients with involuntary leg movements due to myelopathy were studied clinically and polysomnographically. The clinical manifestation and polysomnographical findings of involuntary leg movements were identical to sleep-related periodic leg movement (PLM) (nocturnal myoclonus). Since 2 patients had complete transection of spinal cord due to injury or vascular accident, the spinal cord deprived of supraspinal influences was considered to generate the rhythm of PLM. Suppression of the leg movements during REM sleep was not obvious in the patients with complete transection of spinal cord. In addition, PLM alternated from one side to the other 1-4 times a night with intervals of 1-4 h in all patients. This alternation also seemed to be from the spinal cord. This PLM of spinal cord origin was different from spinal myoclonus in their clinical features although both were generated within the spinal cord. PLM of spinal cord origin showed a triple flexion of the ankle, knee and hip, and this was very similar to a flexor withdrawal reflex which all patients exhibited. Therefore, it was suggested that PLM of spinal cord origin has a common mechanisms with spinal automatism. Although all patients had extensor plantar responses, PLM preceded the paresis in three patients and the severity of paresis was variable. There was no laterality of left and right PLMs even in patients with weakness of the leg on one side. This suggested that PLM of spinal cord origin might be induced by the interruption of the tract which was separate from, but runs near the corticospinal tract.