The thyroid function of 181 patients with Down syndrome was investigated. When compared with a control group of 163 children we found T4 and FT4 levels to be significantly lower and T3 and TSH levels to be significantly higher in the Down syndrome population. Of the 181 patients with Down syndrome, 29 (16%) showed evidence of either uncompensated or compensated hypothyroidism: 11 (6%) had both low T4 and high TSH levels, 14 (8%) had only high TSH values, and 4 (2%) had only low T4 values. One of the patients with Down syndrome had a significantly elevated T4 level. Studying different age groups, we observed a decline of the mean T4, FT4, T3, FT3, and TBG values with advancing age. T4, T3, and TSH blood levels obtained in 1988 were slightly but not significantly lower when compared with values from 1985. Because thyroid dysfunctions in patients with Down syndrome are more common than in the general population, periodic thyroid hormone function tests should be performed in persons with Down syndrome in particular as they advance in age. Thus, individuals with significantly abnormal results can be identified early before clinical symptoms become manifest. If patients with Down syndrome are found to have a thyroid hormone disorder, appropriate treatment should be forthcoming, which in turn will enhance their quality of life.