Two hundred and fifty-four ALS patients from British Columbia, Canada were screened for mutations in the gene encoding the enzyme superoxide dismutase type 1 (SOD1). Thirteen patients (5.1%) carried one of six missense mutations (A4V, G72C, D76Y, D90A, C111Y, I113T). Mutations were found both in sporadic and familial ALS cases. Atypical clinical features delayed diagnosis in some cases. The demographic and clinical features of the mutation carrying index cases are summarized, and compared with those of screened patients without mutations. The phenotypic variability between SOD1 mutation carrying patients in this study is dramatic, even among patients with the same mutation This underlines the hypothesis that ALS is a biologically heterogeneous disorder in which genetics, environment and ageing all interrelate to form the final clinical phenotype.