Out of a population of 138,598 infants born in southern Poland between 1987 and 1989, and screened for phenylketonuria (PKU), 28 cases were ascertained probands and their parents were isolated and eight polymorphic restriction sites were analyzed within the phenylalanine hydroxylase gene region. Twenty-one different haplotypes (HT) were revealed, five of them representing new categories. The most common haplotypes among those carrying normal alleles were: HT1 (27.3%) and HT4 (11.4%). Within the group of haplotypes with mutant alleles the most frequent was HT2 (56.8%), whereas the frequency of this haplotype in other European populations, such as French, Danish and German, ranged from 12% to 24%. HT3, being the most common in Danish (38%), and relatively frequent in the other western European (13-14%) populations, appeared to be very rare in our sample (2.3%). The mutation of codon 408 (exon 12, C----T, Arg----Trp), which has been described to be tightly linked to HT2, was tested on amplified DNA by dot-blot hybridization. This mutation was found in 25 out of 44 proband chromosomes. In one case it was linked to HT5, in the remaining 24 to HT2. Our results confirm molecular heterogeneity of PKU haplotypes, as well as their significant interpopulation variation.