The nuvenile muscular atrophy of unilateral upper extremity was investigated clinically, based on experience with 20 cases. This disorder is not inherited and males are predominantly affected. Juvenile onset, localized muscular atrophy with unique distribution in the arm of one side, and nonprogressive course in later stage are cardinal clinical features of this disorder. The electromyogram demonstrated high-amplitude action potentials with long duration in association with reduction of the total motor-unit voltage during the maximum voluntary contraction of the atrophied muscles. Muscle biopsy demonstrated the picture of muscular atrophy without inflammatory cell reactions. These laboratory data represent the neurogenic nature of the atrophy. This disorder is distinctly different from any hitherto described entities of muscular atrophy, but its clinical features are similar to the anterior tephromalacia of Marie and Foix. Although the clinical picture suggests the lesion in our patients, involving the anterior horn or nerve roots of the lower cervical and upper dorsal segments, the nature of the lesion or the cause of this disorder is still unknown. The final diagnosis should await complete pathologic studies.