A group of 138 community-based patients with Down syndrome were examined for evidence of autoimmune thyroid dysfunction at the time of their referral for routine health care services provided as part of a model program. Twenty-eight patients (20.3%) were found to have previously unrecognized hypothyroidism, and 2 patients (1.4%) had previously unrecognized hyperthyroidism. In addition, 66 patients were tested for thyroid autoantibodies, and 26 were found to have positive antimicrosomal and/or antithyroglobulin antibody test results. There was no statistically significant association between age or sex and the mean thyrotropin value or the presence of thyroid autoantibodies. The relationship between the mean thyroxine level and sex was mildly significant. Of the patients with hypothyroidism, 78.5% were female, and most were between the ages of 30 and 50 years. However, a higher-than-expected number of patients with autoimmune hypothyroidism were under age 30 years. These findings highlight the lack of adequate health care services available to persons with Down syndrome who live in the community. All persons with Down syndrome must undergo regular clinical and laboratory screening for the presence of thyroid disease.