The literature regarding the vitamin B(12) status of patients with phenylketonuria was reviewed. Adequate amounts of B(12) are provided in products used in dietary treatment; however, a number of case reports and cohort studies document deficiency in those who have discontinued taking amino acid, mineral and vitamin supplements but who continue to eat only very limited amounts of natural protein. Symptoms and signs of B(12) deficiency are variable but severe deficiency may cause serious neurological disease. Nitrous oxide anaesthesia is a particular risk. It is recommended that plasma total homocysteine and plasma or urinary methylmalonate should be routinely measured, as they are more sensitive markers of deficiency than serum B(12) concentrations. Functional B(12) deficiency can occur in the presence of a normal B(12) concentration.