Phenylketonuria (PKU), an autosomal recessive disorder, occurs in one of 53,000 births in Korea. The disorder is associated with deficient activity of phenylalanine hydroxylase. In PKU, phenylalanine cannot be used in a normal fashion because of the deficient enzyme. Untreated, affected individuals develop marked mental disabilities, behavioral difficulties, seizures, rashes, pigment dilution, and unusual body odor. PKU treatment consists of a phenylalanine-restricted diet supplemented with a phenylalanine-free mixture of amino acids. During the restricted diet, PKU patients have an increased risk of developing vitamin B12 deficiency because of a limited intake of animal products. We report a successful anesthetic management in a patient with phenylketonuria for dental procedures.