All mutations in the SOD1 gene associated with familial ALS behave as dominant traits. One mutation, however, giving rise to an aspartic acid to alanine substitution in codon 90 (D90A), was reported only to induce motor neuron disease in homozygous individuals in the Scandinavian population. We describe two families with ALS and one apparently sporadic ALS patient who are heterozygous for the D90A mutation. One patient had the unusual phenotype of focal nonprogressing motor neuron disease.