Cystic fibrosis in the northern sector of the French "département" of Finistère is 1:1787 live births. Within this sector a concentration of the disease was found in a small area. The minimal frequency in this area, from 1946 to 1972, was calculated as 1 per 377 live births, the gene frequency being 0.0515. Genealogic analysis, going back to the beginning of the 18th century, showed a relationship between 8 of the 10 families to which the patients belonged. The origin of the deleterious genes may be explained by a least five primary ancestral couples living in the 18th century. Random drift is the most probable explanation for the concentration of cystic fibrosis in this region.