Down syndrome is perhaps the most common genetic condition associated with mental retardation. In cytogenetic examination, trisomy 21 is in 95% of Down syndrome, and the others are mosaicism, translocation or deletion. There are many associated diseases with Down syndrome such as, thyroid function abnormality, congenital heart disease, intestinal blockage, and so on. Hypothyroidism appeared in 15% before adolescent in Down syndrome patients. In Korea, there were several reports of Down syndrome with hypothyroidism in childhood but not in adulthood. And we had three cases of hypothyroidism with Down syndrome in adulthood. Cytogenetic examination revealed trisomy 21 in the 2 cases and 1 case of mosaicism. Antithyroid antibody was positive in one case. None of these cases was admitted due to symptoms of hypothyroidism. It is very difficult to make the diagnosis of hypothyroidism in Down syndrome because of similarity in symptoms between Down syndrome and hypothyroidism. Thus, periodic thyroid function test should be made in Down syndrome, and this could be a part of improving quality of life in Down syndrome.