The known cases of mental retardation which exhibit a genetically determined biochemical lesion were reviewed. Twenty-two inborn errors of metabolism with associated mental defect have been described to date, 12 of these within the past decade. Improved procedures for diagnosis and therapy make this area of investigation a promising one for clinicians, biochemists and geneticists. During a screening program for amino aciduria, a "new" metabolic defect, citrullinuria, was detected in a mentally retarded child. This condition is characterized by the presence of citrulline in the urine, blood and cerebrospinal fluid in concentrations 50- to 100-fold greater than normal. Although the amounts of citrulline excreted appear to be related to the protein intake, it was not possible to reduce the high citrulline concentration in the blood by dietary restrictions.