The exact mechanism of fetal damage in maternal phenylketonuria (PKU) is uncertain and although the fetus is heterozygotic for the gene coding for phenylalanine hydroxylase its immature hepatic enzyme system may be the reason for its inability to deal adequately with transplacental phenylalanine uptake. Several aspects of the management of maternal PKU are discussed and several case studies are presented. Dietary treatment should begin preconceptually despite evidence that post-conceptual treatment can have an acceptable outcome. Maternal recognition of the need for pre-conceptual treatment should increase with improvements in intellectual abilities of PKU girls resulting from neonatal screening and appropriate dietary management.