Hungarian cystic fibrosis (CF) families (n = 33) including 114 family members have been analysed for the presence of the delta F508 mutation within the cystic fibrosis transmembrane conductance regulator (CFTR) gene, and have been haplotyped with probes for restriction fragment length polymorphisms (RFLPs) known to be linked to the CFTR gene. The delta F508 deletion was present in 64% of CF chromosomes. As in many other populations, linkage disequilibrium was found between the CF locus and the haplotype B (XV-2c: allele 1, KM-19: allele 2), which accounts for 95% of delta F508 CF chromosomes in our families.