The distribution of DNA haplotype constellations within the phenylalanine hydroxylase (PAH) gene was investigated in 44 German families affected with phenylketonuria (PKU). The haplotype frequencies differed significantly from those observed in a Danish population. Furthermore, ten haplotypes were identified in addition to the 12 previously described. In one of ten PKU alleles linked to haplotype 3, the G to A transition at the 5' splice donor site of intron 12 could not be confirmed with the use of synthetic DNA probes. According to these data, which are still limited, carrier testing and prenatal diagnosis should be possible in 70% of individuals at risk in the German population.