To investigate this issue, approximately 20 children with mild PKU from 6 to 18 years of age (inclusive) and their parents will participate in the study. The behavior and cognition of children with mild PKU will be assessed using the following methods: (1) Parents will complete inventories to rate the behavior and cognition of their children; (2) Older children will complete self-report inventories to rate their behavior and cognition; (3) Cognitive tasks assessing IQ and executive aspects of attention (i.e., sustained attention and inhibitory control) will be administered to all children.