Prenatal diagnosis of classic phenylketonuria (PKU) was performed after chorionic villus sampling by means of linked restriction fragment length polymorphisms (RFLPs) using the cDNA probe ph PAH 247 (Kwok et al. (1985) Biochemistry, 24, 556-561). We report in this paper a PKU family who were only informative for RFLP analysis by a combination of two RFLPs on the basis of haplotype determination of the normal and mutant phenylalanine hydroxylase (PAH) alleles. The DNA analysis detected a PKU fetus homozygous for mutant PAH alleles and the mother opted for termination in the 12th week of gestation.