The amplification refractory mutation system (ARMS) has been applied to prenatal diagnosis and carrier detection of cystic fibrosis. The nucleotide sequence of both alleles of the PstI restriction fragment length polymorphism at the KM19 locus, which displays linkage disequilibrium with cystic fibrosis, has been determined. ARMS enables direct analysis of alleles of this polymorphism in DNA isolated from chorionic villus biopsy or white blood cells.