As thyroid function has been documented to be of a higher prevalence in individuals with Down's syndrome, a study was set up to assess the thyroid status of these individuals. Thyroid function tests (T.F.T.s) were initially reviewed on 100 individuals with Down's syndrome in the community and on 36 individuals who were residentially based. Abnormal T.F.T.s were then reviewed 3 yr later. In total sample of 136, initially 13 percent [n = 18] of individuals with Down's syndrome had abnormal T.F.T.s, 5 percent [n = 7] were established cases of thyroid disease and 8 percent [n = 11] were newly identified cases who had abnormal T.F.T.s. Three yr later 6.5 percent [n = 9] of the group who had had abnormal T.F.T.s continued to have abnormal T.F.T.s, 5 percent [n = 7] had thyroid disease and 1.5 percent [n = 2] still had biochemical evidence of thyroid dysfunction. There was a statistically significant increase in abnormal T.F.T.s in the residential sample compared to the community sample on both occasions. The incidence of thyroid dysfunction has been found to increase with age, particularly over the age of 40, however in this study the majority were under the age of 40 with an age range between 28.3 yr and 33.8 yr. The results in this study, coupled with the variability of T.F.T.s over time, highlights the need for regular monitoring of the thyroid status of individuals with Down's syndrome.