This is a 10-year multi-center, prospective, longitudinal, single arm study evaluating immunologic, inflammatory and laboratory parameters associated with long-term Palynziq treatment in subjects with phenylketonuria (PKU) in the United States (US). Subjects in the US for whom a clinical decision has been made that they will receive pegvaliase to treat their PKU within 30 days following the date of enrollment in Study 165-501 (incident-users) or who have previously started treatment with pegvaliase at the date of enrollment in Study 165-501 (prevalent-users) are eligible for participation in Study 165-503.