From 1977 through 1981, we examined 23 patients with single-limb atrophy. Thirteen had upper-limb and ten had lower-limb involvement. The characteristic clinical features were insidious onset in the second and third decades, male preponderance, sporadic occurrence, wasting and weakness confined to one limb, and absence of involvement of the cranial nerves, cerebrum, brain stem, and sensory system. The electromyographic features, along with histologic features of neurogenic atrophy, were suggestive of an anterior horn cell lesion. The slow progression of illness for two to four years followed by a stationary phase was observed. There was no clinical evidence of involvement of the other three limbs even in patients with long-standing illness of ten to 15 years' duration.