Cystic fibrosis occurs in 1 in 2000 to 4000 whites, and about 1 in 25 people are heterozygous carriers. The CF gene is cloned, and a single common mutation is found on 70% to 75% of CF chromosomes in most populations. Numerous different mutations are found on the remaining CF chromosomes. DNA analysis for carrier testing should be offered to relatives of CF patients and to reproductive partners of proven carriers. Population based carrier screening could detect about 72% of at-risk couples, and pilot studies are in progress to assess the feasibility of such screening. With regard to these factors, the possibility of gene therapy, and the questions raised, CF serves as a useful model for many genetic disorders.