Five patients with monomelic spinal muscular atrophy are described. Clinical features included insidious onset of wasting and weakness of one limb, lack of involvement of the cranial nerves, brain stem, pyramidal tracts and sensory system, and a stable condition over a period of 4–20 years. Clinical findings, electromyography and/or muscle biopsy were consistent with anterior horn cell lesion. Central cavities were excluded by magnetic resonance imaging studies of the spinal cord. Computed tomography of skeletal musculature and electromyography indicated more diffuse lower motor neuron involvement by revealing abnormalities in clinically unaffected muscles in 4 of the 5 patients. Myokymic discharges were found in the affected limb of 1 patien