['Patients with Down syndrome are prone to autoimmune diseases which also occur in the recessive disease autoimmune polyendocrine syndrome type I (APS I). Since this disease is caused by mutations in the gene AIRE on chromosome 21, one might speculate that altered expression of AIRE contributes to autoimmune disease in Down syndrome.', 'To study the prevalence of 11 well-defined autoantibodies, five of which are specific for APS I, associated with various manifestations of APS I in patients with Down syndrome.', 'Sera from 48 patients with Down syndrome were analysed. Autoantibodies against 21-hydroxylase, 17alpha-hydroxylase, side-chain cleavage enzyme, aromatic L-amino acid decarboxylase, cytochrome P4501A2, tyrosine hydroxylase, tryptophan hydroxylase, glutamic acid decarboxylase 65, tyrosine phosphatase IA-2 and transglutaminase were analysed using an immunoprecipitation assay, and thyroid peroxidase autoantibodies were measured using a haemagglutination assay.', 'Seven of 48 patients had elevated titres of autoantibodies: one against 21-hydroxylase, three against aromatic L-amino acid decarboxylase, one against cytochrome P4501A2, one against glutamic acid decarboxylase 65 and one against tyrosine phosphatase IA-2. None of the patients had clinical or laboratory signs of disease coupled to the respective autoantibody.'] Are autoantibodies linked to autoimmune polyendocrine syndrome type I prevalent in Down syndrome?