The locus D7S23 includes a CpG-enriched methylation-free island that maps midway between the markers J3.11 and met and is genetically very close to the mutation causing cystic fibrosis (CF). We have studied the linkage disequilibrium between four polymorphic markers from this locus (KM.19, CS.7, XV-2c, and PT-3) and the CF mutation (CF) in 127 Italian families. Strong linkage disequilibrium is found between KM.19, CS.7, and CF, and weaker but significant disequilibrium is found between XV-2c, PT-3, and CF. The disequilibrium between markers and CF for the Italian population provides additional information on the origin and homogeneity of the CF defect. This panel of probes is sufficiently informative to permit accurate prenatal diagnosis of CF in most families with an affected person, and the disequilibrium also allows indirect carrier detection/exclusion in some cases.