Phenylketonuria (PKU) is an inborn metabolic disorder, inherited in an autosomal reccesive manner. This genetic defect results in missing or defective in phenyloalanine hydroxylase activity (PAH)- a liver’s enzyme which converts the essential amino acid, phenyloalanine (PHE) into tyrosine. Only about 3% of cases have deficiency in synthesis or recycling of the enzyme’s cofactor, tetrahydrobiopterin. As a result of these abnormalities serum fenyloalanine levels and its metabolites become elevated and harm the developing central nervous system. In affected children it leads mostly to mental retardation, but also to variety of neurological problems. If the babies with PKU are identified through new born screening programs and immediately started PHE – restricted diet is conducted, the children are able to achieve normal development and intelligence. In Poland the incidence of phenylketonuria is 1: 7000 of newborns, in other words, about 60 children are born per annum and every 46 healthy person is a carrier of mutation in the PAH gene. Recently, one of the most disturbing issue, due to increasing population of women with PKU reaching the procreation age, is maternal phenyloketonuria (MPKU). MPKU manifests in embryopathy and fetopathy, which is related to the remaining elevated concentration of maternal blood phenyloalanine level. In order to preserve the great advantages of screening programs, there is a necessity to maintain a preventive treatment particularly in affected women.