A sample of 175 Italian cystic fibrosis patients has been analysed for the presence of the delta F508 mutation. The frequency of this mutation among 137 patients with pancreatic insufficiency is equal to 57%; in 23 patients with pancreatic sufficiency it is 26%. A high proportion of the unknown mutations is associated with the same rare haplotype found in association with delta F508, suggesting that at least another mutation occurred on a chromosome characterized by the same haplotype.