BACKGROUND: Juvenile focal amyotrophy of the arm is an unusual focal motor neuron disease that is rarely familial. Its pathogenesis is unknown. We recently described a family with amyotrophic lateral sclerosis associated with a mutation in the superoxide dismutase 1 (SOD1) gene substituting an aspartate for an alanine (D9OA). One of the carriers of this mutation had focal and nonprogressive amyotrophy of the arm, suggesting that focal amyotrophy might be associated with SOD1 mutations. OBJECTIVES: To describe the phenotype of 2 brothers with juvenile focal amyotrophy of the upper extremity and to characterize their SOD1 genotype and SOD activity. METHODS: Polymerase chain reaction and sequencing of the SOD1 gene and colorimetric measurement of the enzyme activity. RESULTS: We compared the phenotype of our patients to that of 375 patients described in the Western literature. The 5 exons of the SOD1 gene were normal, as was the SOD activity in red blood cells.