The frequency of six common mutations in the cystic fibrosis transmembrane conductance regulator gene was studied in 100 patients hospitalized with chronic bronchitis. Only one patient with chronic bronchitis and diffuse bronchiectasis was heterozygous for the common delta F508 mutation. R553X, G542X, G551D, N1303K and 621 + 1G-->T were not detected. This result is not significantly different from the frequency of cystic fibrosis carriers in Northern Europe. Predisposition of heterozygotes for chronic bronchitis is therefore unlikely.