Standardised intellectual, motor and behavioural tests have been undertaken in a group of 73 children and 43 adults with congenital hypothyroidism in a regional study in the north of England. These provide comprehensive data with which to compare the results of studies of children diagnosed by screening and indicate the degree of abnormalities persisting into adult life in those patients diagnosed before screening was introduced. The mean IQ scores of the hypothyroid children and adults were 1-2 standard deviations (SD) below the population means. Longitudinal IQ data indicate persistence of intellectual abnormalities, though there was an increase in IQ score in several patients. The mean motor scores were reduced by 1-2 SD in the children and 1-3 SD in the adults. Behavioural problems and personality difficulties mainly of a neurotic nature also persisted into adult life.