An inborn error of metabolism in which affected individuals lack the liver enzyme phenylalanine hydroxylase (PAH), which is needed to metabolize phenylalanine, an amino acid essential for normal growth and development. If untreated, affected individuals may become severely mentally retarded, become microcephalic, have behavioral problems, develop epilepsy, or show other signs of neurological impairment. Phenylketonuria (PKU) is inherited as an autosomal recessive trait and is found in all ethnic groups but most frequently in individuals of northern European descent. Its incidence is about 1 per 14,000 births in the United States. Classically, persons with phenylketonuria exhibit blood phen­ylalanine concentrations of 20 mg/deciliter or more (normal concentrations are about 1–2 mg/dl), normal blood tyrosine levels, and excessive phenylalanine metabolites in the urine while on a normal diet. Phenylketonuria variants have blood phenylalanine concentrations of 10–20 mg/dl, but may not have phenylalanine metabolites in their urine unless they have ingested excessive amounts of protein.