The proposed study is designed to evaluate the hypothesis that functional connectivity may represent an early marker for neurocognitive improvements related to Kuvan treatment. As a first step in the study, 20 patients with PKU who are ≥ 6 years of age will receive baseline functional connectivity magnetic resonance imaging (fcMRI) evaluations before being treated with Kuvan™. Response to Kuvan™ will be monitored for 4 weeks. At the end of 4 weeks, response to Kuvan™ will be reviewed and all 20 patients will receive follow-up fcMRI evaluations. Fifteen patients with a reduction of ≥ 20% in blood phenylalanine will continue in the study and receive a third fcMRI evaluation at the end of 6 months of treatment with Kuvan™. For comparison purposes and to control for possible practice effects in repeated testing, a matched control group of 20 healthy individuals without PKU will used. All 20 of the control subjects will receive baseline and 4-week fcMRI evaluations; fifteen will receive a 6-month evaluation.