SUMMARY: Though the investigators know that KuvanTM lowers blood Phe levels and improves tolerance for natural protein in at least half of the PKU (Phenylketonuria) patient population, investigators do not know the full effects this medicine will have on the patient's diet, or what impact the medicine or diet changes will have on the body composition or nutrient status of PKU patients. Since KuvanTM may also help the body produce neurotransmitters, investigators also want to find out if taking KuvanTM changes neurotransmitter levels in PKU patients, and if PKU patients who are benefitting from KuvanTM feel less stigmatized and have a better outlook on life as a result of the treatment.