CF heterogeneity has been evidenced from both clinical and genetic observations. At least two clinical forms of CF are easily distinguishable: CF with meconium ileus and CF without meconium ileus. The results of prenatal diagnosis have shown that the recurrence rates of CF are different in these two clinical forms. Molecular analysis of Restriction Fragment Length Polymorphisms (RFLPs) tightly linked to the cystic fibrosis (CF) gene defined several types of CF and normal chromosomes in a French sample of 64 families with CF. The CF mutation is tightly linked to one XV-2C and KM19 RFLPs haplotype but is differently linked to J3.11 RFLP alleles, depending on whether or not the clinical form of CF is associated with ileus. A distortion of the segregation ratio observed between normal and CF haplotypes in the families with ileus could explain the high recurrence rate of CF in such families.