A wide range of clinical phenotypes are associated with mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. In some cases, CFTR mutations have been identified in adults presenting with isolated features of cystic fibrosis such as mild lung disease, pancreatic insufficiency or male infertility, with normal sweat duct function. Nasal polyps are also a prominent feature of cystic fibrosis but it is unknown whether they also may represent a monosymptomatic form of the disease. We have screened the CFTR gene of 55 patients with severe nasal polyposis, who had no other features which would suggest a diagnosis of cystic fibrosis. The screening looked for mutations common to 86% of sufferers from cystic fibrosis. Three carriers were detected including one patient with the uncommon R117H mutation and two carriers of the delta F508 mutation. In addition there was one presumed carrier with a cystic fibrosis child, whose mutation we did not detect. This study has identified three CFTR mutation carriers in 55 patients with severe nasal polyposis. This is not significantly higher than the incidence in the general population. We cannot, from these figures, categorically rule out a slight association between CFTR mutations and simple nasal polyposis. However, we have demonstrated that the vast majority of patients with polyposis do not have inactivation of the CFTR gene.