The primary objectives are two-fold. First, we will determine if behavior and cognition are compromised in children with mild PKU prior to treatment with Kuvan (baseline). To accomplish this objective, we will administer measures of behavior and cognition that include normative data based on age. We hypothesize that children with mild PKU will have ratings and scores that are ≥ 1 standard deviation from the normative mean. Second, we will determine if behavior and cognition improve in children with mild PKU following treatment with Kuvan. To accomplish this objective, we will administer the same measures of behavior and cognition after 4 and 24 weeks of treatment with Kuvan(4-week and 24-week follow-ups, respectively). We hypothesize that the follow-up ratings and scores of children with mild PKU will improve by ≥ 0.5 standard deviation relative to their baseline ratings and scores.