We observed 37 patients (mean age at onset, 48.5 years; range, 13 to 84 years) with segmental myoclonus (18 branchial, 19 spinal). Etiologies for branchial myoclonus included brain-stem demyelination, cerebrovascular disease, Meige's syndrome, cerebral arteritis secondary to bacterial meningitis, central nervous system Whipple's disease, acute cervicomedullary trauma, and cerebellar degeneration. Spinal myoclonus was associated with laminectomy, remote effect of cancer, spinal cord injury, post-operative pseudomeningocele, laparotomy, thoracic sympathectomy, poliomyelitis, herpes myelitis, lumbosacral radiculopathy, spinal extradural block, and myelopathy due to demyelination, electrical injury, acquired immunodeficiency syndrome, and cervical spondylosis. The latency between the predisposing condition and the onset of myoclonus ranged from immediate to 33 years (mean, 2.9 years). In six patients, the myoclonus was the presenting symptom of a serious underlying disease. Treatment with clonazepam, tetrabenazine hydrochloride, or other medications provides a satisfactory control in most patients.