Severe mental retardation due to maternal phenylketonuria.

PHENYLKETONURIA (PKU) which, in Northern Ireland, has an incidence of one in 4362 newborns,' is a metabolic disorder in which the conversion of phenylalanine to tyrosine is impaired due to the reduction in activity of the enzyme phenylalanine hydroxylase. It is inherited an an autosomal recessive trait. If begun in early infancy, a low phenylalanine diet which controls the biochemical abnormalities prevents severe mental retardation. However, since j9572 it has been recognised that infants born to women with PKU have a high risk of mental retardation, microcephaly, congenital heart disease, and low birth weight. We report a family in which two sisters, both with unrecognised PKU had a total of six children, all of whom had microphaly with mental retardation.