We report on an Austrian pedigree with autosomal dominant amyotrophic lateral sclerosis (ALS), diagnosed in six patients from two generations. The only surviving clinically affected family member was examined in our ALS clinic. Historical information on other affected individuals was obtained from knowledgeable family members. The mean +/- S.D. age of onset of the disease was 54 +/- 6.9 years, with a range of 43-66 years. The duration of the index patient's disease until death was 8 months. Using single strand conformational polymorphism (SSCP) analysis, we studied the index patient's exons 1, 2 and 4 of the Cu/Zn superoxide dismutase gene (SOD1) on chromosome 21. A variant banding pattern was observed for exon 1. Sequencing studies showed a previously undescribed T to A missense mutation at position 8 in exon 1 of the SOD1 gene. This mutation results in the elimination of an Eco57I restriction site. Whereas the index patient was heterozygous for this restriction site, 50 unrelated healthy controls and an unaffected brother were not. The mutation lies in a region involved in dimer contact in the three-dimensional structure of the SOD1 protein. This region comprises other known sites for ALS-causing mutations.