Mutations of the copper/zinc superoxide dismutase (SOD-1) gene are present in around 20% of patients with a family history of amyotrophic lateral sclerosis. The finding of these mutations in patients with sporadic amyotrophic lateral sclerosis is rare. We describe a family with amyotrophic lateral sclerosis associated with the SOD-1 mutation Asp 101 Asn. This mutation was previously described as occurring in a patient with sporadic disease. We discuss the difficulties in defining truly sporadic amyotrophic lateral sclerosis, and the consequent implications on the neurogenetic advice given to other family members.