A discription is given of 24 patients with the benigen focal amyotrophy; Nineteen had upper-limb and five had lower-limb involvement. The characteristic clinca! Features were insidious onset in the second and third decades, male predominance. Sporadic occurance, wasting and weakness confined to one limb, relatively good prognosis and absence of involvement of the cranial nerves, cerebrum, brainstem, and sensory syetem. The nerve conduction studies, electromyographic pattems, somatosensory evoked potential studies, and CTMM or MR findings suggested anterior horn cell disorders. In nine cases we were able to demonstrate focal atrophy in the lower cervical cord limited to the ipsilateral anterior horn region, and two cases segmental cord atrophy in the lower cervical cord in MR or CTMM studies. The benign focal amyotrophy, for which no causes been described, is benign and self-limiting, unlike most motor neuron diseases.