Size and distensibility of large airways have important implications for flow limitation and the efficacy of coughing. From radiological and functional data, some authors have suggested an increased size and distensibility of the trachea in cystic fibrosis (CF). Using computed tomography (CT) we compared size and distensibility of the trachea in 5 cystic fibrosis patients and five age- and height-matched healthy volunteers. Tracheal cross-sectional area was measured 25 mm below the cricoid cartilage. CT recordings were made at functional residual capacity, at 0 and +20 cmH2O mouth pressure. Inductive plethysmography was used to check that during these manoeuvres lung volume did not change and that the glottis remained open. Tracheal cross-sectional area and derived indices of tracheal distensibility were similar in the two groups. This study does not support the concept of an increased size and distensibility of the trachea in cystic fibrosis.