Axial myoclonus of propriospinal origin. Three patients are described with nonrhythmic repetitive axial myoclonic jerks causing symmetric flexion of the neck, trunk, hips and knees. No electrophysiological evidence of a cortical or brainstem reticular origin for the myoclonus was found. In the first patient the axial jerks only occurred spontaneously. The latencies of recruitment of spinal segments during a jerk indicated that the discharge arose in the mid thoracic cord and then slowly spread at about 5 ms-1 up and down the cord to involve rostral and caudal segments. No structural lesion was identified in this patient. In the second patient spontaneous and reflex axial jerks developed following the excision of a cervical haemangioblastoma. In the stimulus-induced jerks the relative latencies of muscles innervated by rostral and caudal spinal segments suggested that the myoclonus originated between the upper cervical and midthoracic cord. In the final patient, EMG activity during spontaneous and stimulus-induced jerks commenced in the rectus abdominis, and was followed by later activity in muscles innervated by rostral spinal segments, suggesting that the myoclonus originated in the midthoracic cord. No structural lesion was identified in this patient. Electrophysiological evidence is used to argue a spinal origin for these axial jerks in all 3 cases. There are striking features common both to this form of human myoclonus and to long propriospinal pathways identified in animals. The new concept of propriospinal myoclonus is suggested. 