To validate the outcome measures and the tolerability of Kuvan treatment in the improvement of behavioral symptoms in 10 selected adults with Phenylketonuria (PKU) with or without mental retardation.

Inclusion Criteria:

• Biochemical confirmation of PKU,
• Measurable maladaptive behavior at baseline,
• Signed informed consent,
• Not pregnant or lactating,
• Females on birth control if applicable,
• Willing to comply.

Exclusion Criteria:

• Subjects or care provider unreliable, use of investigational products within 30 days prior to enrollment,
• Pregnant or lactating,
• Concurrent disease or condition that would interfere with study participation or safety, ie multiple psychotropic meds, any condition the PI renders the subject at high risk, on drugs that inhibit folate synthesis,
• Concurrent use of levodopa.

Individuals with PKU followed at our clinic