Cystic fibrosis is a common autosomal recessive disease in white persons. Prenatal diagnosis by DNA analysis became possible in families with a child who is affected by cystic fibrosis when the probes pJ3.11, metH and metD, which are linked closely to the cystic fibrosis gene (CF) were described. The recent description of the XV-2c and KM.19 probes has improved the prenatal diagnosis of cystic fibrosis greatly. The KM.19 probe alone was informative in eight of 12 families that were studied while XV-2c was informative in eight of 12 families that were studied while XV-2c was informative in only two of the 12 families. In contrast, the use of the pJ3.11, metH and metD probes in combination allowed full diagnosis in six of the 12 families. The combined use of the CF-linked probes produced informative data for all 12 families. Therefore, in most families with at least one affected living child, the first-trimester diagnosis of cystic fibrosis is possible with fetal DNA that has been prepared from chorionic villous samples. Strong linkage disequilibrium was found with both the KM.19-PstI polymorphism and the XV-2c-TaqI polymorphism and the CF gene.