A direct mutational analysis of the phenylalanine hydroxylase gene (PAH) in Gypsy families with phenylketonuria (PKU) has not yet been presented. However, they obviously represent a group at high risk for this inherited disease. We analyzed the PAH loci of 65 Gypsies originating from Eastern Slovakia by a combination of PCR amplification, direct sequencing and ASO hybridization. These studies uncovered 10 "classical PKU" patients to be homozygous for a R252W (CGG-TGG) transition, and 29 heterozygous carriers of this mutation. Fifteen control Caucasoid PKU patients from the Czech and Slovak Republics were selected. In this group we detected R252W mutation in two subjects (6.67% of all mutant alleles). Both were compound heterozygous for two different mutations. Previous haplotype studies of Welsh Gypsies with PKU were uninformative in the determination of heterozygosity. ASO hybridization served us effectively for the consequent analyses in Gypsy PKU-related families and to identify the carriers among the unrelated subjects.