• Neuropsychological abilities at 1,3,5, and 6 years of age were compared in 107 children with congenital hypothyroidism identified by newborn screening who had been either breast-fed or formula fed during infancy. The thyroxine level was significantly higher in breast-fed children than formula-fed children at 1 and 2 months of age, but the level was not higher later during the first year of life. Controlling for parent IQ, socioeconomic status, and dose level, no differences in overall neuropsychological functioning were observed. Children with ectopic glands showed a significant advantage with breast-feeding on several tasks at 3 years of age. These results signify that thyroxine levels in infancy from breast-feeding are not necessarily reflected in improved intellectual functioning later in childhood.