Seventy-one cases of distal and segmental muscular atrophy of the upper extremities with juvenile onset were studied. The clinical features consisted of: juvenile onset, male preponderance, unique distribution of the muscular atrophy in the hand and forearm, tendon reflexes hypoactive in most cases but hyperactive in some, no definite sensory disturbances, no involvement of the cranial nerves, and autonomic nerve disorders in the affected region. There was rapid progression during the 2 to 3 years after onset with a slowly progressive course thereafter. There were no abnormal laboratory findings except for electrophysiological and morphological findings of the affected muscles. The site of lesion was surmised to be from the C5 to T1 spinal segments with intramedullary involvement. The cause is unknown.