Phenylketonuria (PKU) is an autosomal recessive disorder resulting from a deficiency of phenylalanine hydroxylase, which converts phenylalanine to tyrosine. Phenylalanine hydroxylase is one of the three aromatic amino acid hydroxylases that utilizes tetrahydrobiopterin (BH4) as cofactor. The published reports indicate that there is altered energy metabolism in the brain of patients with PKU. Phenylalanine and its metabolites appear to impair several aspects of brain energetics including: (1) Inhibition of glucose uptake; (2) diminished glycosylation of cytoskeletal proteins; (3) Inhibition of pyruvate kinase; and (4) reduced flux through the glycolysis. Studies in vivo with magnetic resonance spectroscopy have demonstrated phenylalanine-responsive abnormalities in cerebral energy metabolism.