Maternal PKU is an embryo-fetopathy caused by elevated plasmaphenylalanine levels in pregnant women with hyperphenylalaninemia and phenylketonuira. Leading symptoms are microcephaly, mental retardatioin and congenital malformations, especially congenital heart disease. Maternal PKU becomes more important since early treated and normally developed girls with PKU are reaching their reproductive age in increasing numbers. There is a lack of adequate knowledge about the dangers of maternal PKU in at-risk women. Only 43% of these women in the Federal Republic of Germany are located by now and can be informed and instructed. Ways and conditions of tracking are described.