In phenylketonuria (PKU) usually there is a defect in the phenylalanine hydroxylase (PAH) gene. Eight restriction fragment length polymorfisms (RFLP's) in the PAH gene together constitute the haplotype. A considerable number of mutations, responsible for the gene defect, some of which are rather frequent, have been described. Here, we present the first results of investigations on the distribution of haplotypes and mutations in PKU patients in the Netherlands. A short literature review is presented.