Twenty early-treated children with classical phenylketonuria (PKU), five early-treated children with variant PKU and seven untreated children with hyperphenylalinemia were compared with non-PKU family members in terms of intellectual development, and 14 school-age PKU children were also compared for academic achievement. For the early-treated children with classical PKU, mean IQ (98) was within the normal range, but nine of these 20 children had IQ scores more than 1SD below those of family members. There was a significant negative correlation between phenylalanine concentrations at one to four years of age and later measured intelligence in these early-treated children, but this was probably a consequence of poor dietary control in the early years. The early-treated children with variant PKU and those with hyperphenylalaninemia had IQ scores consistent with those of unaffected family members, but untreated children with variant PKU had scores significantly lower than their own early-treated siblings. Achievement scores of the early-treated PKU children were consistent with their intellectual ability: they and their non-PKU siblings had similar standard scores for reading and spelling, but arithmetic scores were significantly lower for the PKU children. Early-treated children whose diet had been discontinued had achievement scores in all subjects below those predicted from their IQS.