Recent reports have shown heterozygosity for some twenty different mutations in the CuZn–superoxide dismutase (CuZn–SOD) gene in familial amyotrophic lateral sclerosis (FALS), and analysed samples from patients have shown decreased enzymic activity. Here we report homozygosity for an exon 4 mutation, Asp90Ala in fourteen patients among four unrelated ALS families and four apparently sporadic ALS patients from Sweden and Finland. The erythrocyte CuZn–SOD activity is essentially normal. Our findings suggest that this CuZn–SOD mutation causes ALS by a gain of function rather than by loss, and that the Asp90Ala mutation is less detrimental than previously reported mutations.