Using heteroduplex analysis generated with synthetic PCR-amplifiable DNA we have screened the PKU populations of southwest England and Wales, western Scotland, and southeast and central England for mutations in exons 3, 7 and 12 of the phenylalanine hydroxylase (PAH) gene. The technique characterized three mutations in exon 12, two in exon 3 and five in exon 7. Altogether over 370 PKU chromosomes were screened. In all geographical regions exon 12 mutations (R408W, IVS12nt1g- > a and Y414C) accounted for about 40% of mutant chromosomes. Exon 3 mutations (principally I65T) were found on between 9 and 12% of mutant alleles and exon 7 mutations accounted for a further 5-7%. Heteroduplex analysis is rapid, simple and safe and three constructs covering three exons can identify between 55 and 60% of mutations in various PKU populations of the UK.