Strong linkage disequilibrium (LD) was found between DNA marker XV2c and the cystic fibrosis (CF) locus (delta= 0.46) and between DNA marker KM19 and CF (delta= 0.67) in 157 CF and 138 normal chromosomes from US Caucasians. DNA haplotypes with nine polymorphic sites were created in 54 Caucasian families. There is a strong LD between the haplotypes and the presence of the mutant CF genes. This implies that the DNA polymorphisms examined are close to the CF gene and that one mutation of the CF gene predominates in the Caucasian population. Haplotype analysis can also be used to refine estimates of CF carrier risk in Caucasians. Data for XV2c and MET markers in 16 American black patients and their families revealed a different haplotype distribution and LD pattern with the CF locus. These data suggest that racial admixture alone does not explain the occurrence of CF in American blacks and that multiple alleles of the CF gene may exist in this population.