Single base substitutions have been identified in two mutant phenylalanine hydroxylase (PAH) alleles that cause phenylketonuria (PKU). The two mutant alleles are common among caucasians of northern European ancestry; detection in genomic DNA samples of patients and carriers by hybridisation with oligonucleotides specific for the respective mutant alleles requires fractionation of restriction-enzyme-digested genomic DNA samples by gel electrophoresis. This method is too cumbersome for mass screening of PKU carriers. Identification of carriers of the mutant alleles was achieved by direct analysis of their genomic DNA samples after specific amplification of a sub-genomic DNA fragment containing both mutation sites by polymerase chain reaction. The results suggest that it is technically feasible to develop a programme for carrier detection of the genetic trait in the population for individuals without a family history of PKU.