Familial amyotrophic lateral sclerosis can be related to mutations in the Cu/Zn superoxide dismutase gene (SOD1) located on chromosome 21q22.1. This is the first report of a SOD1 mutation in a patient with Down syndrome. A 34-year-old woman with Down syndrome developed a lower motor neuron disease that led to death in two years. Autopsy findings confirmed the diagnosis. DNA examination identified a missense mutation at nucleotide 134 of exon 5 of the SOD1 gene resulting in the aminoacid substitution serine-134-asparagine (S134N). A real time PCR detected the mutation in two out of three alleles. The 70-year-old mother of the patient carries the same mutation but has not yet developed the disease. The missense mutation of SOD1 gene in two of the three alleles could have increased its toxic effects in the Down syndrome patient leading to an earlier onset and rapid progression of the disease.