As a result of recent advances in molecular genetics, carrier testing for cystic fibrosis (CF) is now available as a clinical assay in a limited number of laboratories. Because neither the gene nor the mutation for this disorder has yet been defined, the analysis relies on an indirect approach that uses DNA sequence polymorphisms and linkage analysis. With use of this general approach, several family members, in addition to those persons seeking carrier information, must be tested. Currently, more than 97% of families are "fully informative" when these markers are used in a linkage analysis; thus, the carrier status of most persons who have a relative with CF can be determined. Recently, strong linkage disequilibrium has been shown between two polymorphic loci (defined by the DNA probes KM.19 and XV-2c) and the CF locus. Because of this important finding, haplotype testing can be used in many clinical settings, such as for families in which a DNA sample is not available from the affected person or for those families in which one spouse has no family history of CF and the other is either affected or is at a high risk of carrying the CF mutation. Overall, the application of recombinant DNA techniques has greatly enhanced the ability to determine, with a high level of accuracy, the carrier status of those persons at risk for inheriting the CF mutation.