The respiratory epithelium of large airways was studied in biopsy specimens obtained by bronchoscopy in seven patients with cystic fibrosis (CF). All but one of these patients were chronically colonised with Pseudomonas aeruginosa. Biopsy specimens were also obtained from the large airways of 10 control patients with or without signs of chronic bronchitis. All samples were studied by transmission electron microscopy. Morphometry was applied to assess the volume density of various cell types and intercellular spaces and for quantification of microvilli of ciliated cells. In CF patients, the average number of microvilli per ciliated cell was decreased by 17% compared to patients with chronic bronchitis and by 34% compared to patients with apparently normal bronchi (p less than 0.02, respectively), but otherwise no abnormalities were found. In particular, the volume density of goblet cells was not increased. Thus the ultrastructure of the airway epithelium may be nearly normal in CF, even in adult patients with chronic respiratory infection.