The analysis of 21 families affected with classical phenylketonuria (PKU) from the Moravian area of Czechoslovakia has revealed 12 different RFLP haplotypes. Nine and eight haplotypes were associated with the normal and with the mutant alleles, respectively. Most normal alleles are associated with haplotype 1 (42.9%). Almost 80% of all mutant alleles are confined within only three haplotypes (1, 2 and 4). There was a strong association between haplotype 2 and the Czech mutant alleles (61.9% of the mutant alleles compared with 4.8% of the normal alleles). There was linkage disequilibrium between this haplotype and the R408W mutation in exon 12. Two mutant haplotypes 7 were found and in both cases they were tightly linked with G272ter mutation. Our finding is in agreement with observations in other Eastern European countries. These data provide further support for the theories of the spread of the R408W mutation from east to west in European populations.