Previous research supports the notion that children with early-treated classical phenylketonuria (PKU) have specific cognitive deficits in executive function skills. These deficits may relate to depressed levels of dopamine, due to defective tyrosine synthesis. We investigated whether deficits reported for preschoolers with early-treated PKU are manifested also among school-age children with PKU, and whether cognitive performance among the latter group is related to phenylalanine level at time of testing. Seventeen children with PKU and 17 age-, sex-, and IQ-matched controls were tested individually on measures of executive functions. The results demonstrate that executive function deficits reported for preschoolers with PKU were not manifested in the school-age PKU children included in this study. The authors discuss the implications of these findings for theories of the development of executive function skills. © 1994, Taylor & Francis Group, LLC. All rights reserved.