AbstractOur experience to date with 40 pregnancies in 13 women with hyperphenylalaninemia indicates that to prevent mental retardation in their children, maternal phenylalanine levels of 120–600 umol/L are needed, in addition to a newborn screening program. While treatment results to date are preliminary, it appears that the phenylalanine‐restricted diet is of value and should be instituted as soon as pregnancy is planned by phenylketonuric women. There is much to be learned by study of these women and referral to a collaborating center in the National Maternal PKU Collaborative Study is recommended.