Thirty-nine children at 7 to 9 years of age with congenital hypothyroidism have been studied with respect to their intellectual, neurological and social functions. Their intellectual achievement as assessed by the WISC test, was significantly lower than in a reference population (mean IQ 88, range 50-113). In children who showed signs and symptoms of hypothyroidism during their first four weeks of life, there was a negative correlation between the age when therapy was started and intellectual development. No such correlation was found in children who showed signs and symptoms after the first four weeks of life. Nine children of 26 with signs of hypothyroidism in the neonatal period, were found to have neurological abnormalities consisting of fine and gross motor disturbances. The neurological abnormalities seriously affected daily life in only the two most severely mentally retarded. No correlation was found between the age when the therapy was started and neurological abnormalities. Three children attended special schools because of mental retardation and six attended normal schools but required additional teaching assistance for learning disabilities. Our results suggest that the risk of future intellectual handicaps may be reduced by early treatment in children with hypothyroidism presenting during the neonatal period.