The extent to which autoimmunity contributes to thyroid dysfunction in individuals with Down syndrome (DS) has not been clarified. In this study, we used the same highly sensitive method to detect both thyroid autoantibodies (thyroglobulin and thyroid peroxidase autoantibodies) in 70 children (32 M and 38 F) with DS, mean age 10.5 y (range 1-19 y). Twenty-seven (39%) of the patients were found to have thyroid autoantibodies, the prevalence of antibody positivity increasing with age. Of the 17 (24%) of the series who were hypothyroid (i.e. high basal TSH level and a low total- or free-T4 level), 11 had thyroid autoantibodies, and another 6 with thyroid autoantibodies became hypothyroid during 13-35 months of follow-up. Thus, the findings suggest that the majority of hypothyroid children with DS suffer from autoimmune thyroid disease.