In order to establish counseling guidelines and aid carrier risk assessment, we sought to establish the frequencies of cystic fibrosis (CF) mutations that are present in CF families living in the Ile de France region, a region notable for its ethnic heterogeneity. We studied 470 CF chromosomes in which we identified more than 90% of the CF mutations. We systematically screened 21 exons and the adjacent sequences of the CF transmembrane conductance regulator (CFTR) gene by denaturing gradient gel electrophoresis using chemical clamps. We detected 41 different CF mutations located in 14 exons. One of these mutations had not been previously described. Given the heterogeneity of these mutations, population screening does not appear to be readily feasible in our population.