Thyroid function has been studied in 121 patients between 13 and 48 years old with proven Down syndrome. Chemically, hypothyroidism was found in 17% and hyperthyroidism in 2.5% of the patients; 18% of patients had goiter. Thyroid antibodies were detected in 33% of subjects studied. The abnormal findings were almost equally distributed between the sexes above 20 years of age; however, females had significantly more thyroid disease than males in the younger age group.