This study of 111 institutionalised Down's syndrome subjects and 74 controls found a relatively low risk of hypothyroidism in both groups compared with certain other studies of Down's subjects, which found very high rates of thyroid hypofunction. However, the Down's syndrome group had a significantly raised risk of hypothyroidism (9%), hyperthyroidism (1.8%) and the presence of thyroid microsomal antibodies (29%) compared with the controls and with laboratory reference data. It is recommended that thyroid function testing should be routine in older Down's syndrome subjects, with special attention being given to the clinical status of those with microsomal antibodies present.