Mutations in the Cu/Zn superoxide dismutase 1 (SOD1) gene have been reported to cause adult-onset autosomal dominant amyotrophic lateral sclerosis (FALS). In sporadic cases (SALS), de novo mutations in the SOD1 gene have occasionally been observed. All the SOD1 mutations are autosomal dominantly inherited with the exception of D90A. To date, in Italy, only two sporadic ALS cases carrying the D90A mutation have been reported in a homozygous state. We investigated for the presence of this mutation in 169 unrelated ALS patients from southern Italy. The genetic analysis revealed three ALS patients (1.8%) with mild phenotype carrying the homozygous D90A mutation.