Mass population screening of newborn infants for congenital hypothyroidism was introduced in 1974 and now is a routine and effective means of early diagnosis of congenital hypothyroidism throughout most of the industrialized world. A large number of affected infants and children have been treated with replacement thyroid hormone, and several reports of IQ measurements and functional assessments of 5-to 7-year-old treated children now are available. These reports document normal mean IQ values, satisfactory school performance, and minimal motor dysfunction in treated children. However, there have been reported correlations between lower IQ values and biologic parameters of the hypothyroid state in the neonatal period among several reported studies, and it is not yet clear whether early adequate treatment will reverse all of the effects of congenital hypothyroidism.