A total of 140 cases of cystic fibrosis in 110 families was identified in Israel during the years 1946--75, and there was reason to suspect inadequate ascertainment. Cystic fibrosis is frequent among the Ashkenazic and Arab populations with an incidence approximating that for Caucasians of European ancestry (1:1,800 to 1:4,000 live births). It is about half as frequent in the Sephardim, rare in the Iraqi or Yemenite Jews and perhaps absent in Iranian Jews. The clinical picture of cystic fibrosis in Israel in all the ethnic groups was found to be the same as elsewhere in the world. In-depth.interviews of 50 families were conducted to assess their understanding of the genetics of the disease and their attitude toward family planning.