The thyroid function and health status of 101 children with Down syndrome were assessed annually for five years. One child had congenital hypothyroidism at entry to the study. During the study period, eight more developed compensated hypothyroidism. Five of 10 children with compensated hypothyroidism still had the condition at the end of the study, it resolved spontaneously in four and one child developed uncompensated hypothyroidism. There were no significant differences in growth and development between those with compensated hypothyroidism and those with normal thyroid function. Two children developed transient rises in thyroxine, associated with elevations in thyroid-stimulating hormone (TSH). A large proportion of thyroid dysfunction in children with Down syndrome is transient and may be related to inappropriate secretion of TSH or thyroid insensitivity to TSH, rather than to auto-immune thyroiditis.