Phenylketonuria (PKU) was the first inherited metabolic disease identified by newborn screening and treated with diet to prevent the development of intellectual disability. Classification of the severity of phenylketonuria is based on the genetic mutation, dietary phenylalanine tolerance, and pretreatment blood phenylalanine concentrations. The etiology of brain damage in PKU has not been fully elucidated; however, high blood phenylalanine concentrations are associated with changes in brain morphology (gray and white matter) and decreased neurotransmitter synthesis.