Sub-study Inclusion criteria same as main study:

• ≥ 8 years of age
• Confirmed diagnosis of PKU
• Willing to continue current diet (typical diet for the 3 months prior to study entry)unchanged while participating in the study
• Willing and able to provide written, signed informed consent or in the case of subjects under the age of 18, provide written assent (if required) and written informed consent by a legally authorized representative after the nature of the study has been explained, and prior to any research-related procedures
• Sexually active subjects must be willing to use an acceptable method of contraception while participating in the study and for at least 30 days following the last dose of sapropterin dihydrochloride
• Females of childbearing potential must have a negative pregnancy test at screening and be willing to have additional pregnancy tests during the study. Females considered not of childbearing potential include those who have been in menopause at least 2 years, or had tubal ligation at least 1 year prior to screening, or have had total hysterectomy
• Willing and able to comply with all study procedures

All other sub-study Exclusion criteria same as main study:

• Has known hypersensitivity to sapropterin dihydrochloride or its excipients
• Subject breastfeeding at screening or planning to become pregnant (subject or partner) at any time during the study
• Use of any investigational product or investigational medical device within 30 days prior to screening, or requirement for any investigational agent prior to the completion of all scheduled study assessments
• Received sapropterin dihydrochloride within 16 weeks of randomization • Have initiated or adjusted medication for treatment of ADHD, depression, or anxiety ≤ 8 weeks prior to randomization
• Taking medication known to inhibit folate synthesis (eg, methotrexate)
• Any condition requiring treatment with levodopa or any PDE-5 inhibitor
• Concurrent disease or condition that would interfere with study participation, compliance or safety as determined by the Investigator
• Any condition that, in the view of the Investigator, places the subject at high risk of poor treatment compliance or not completing the stud