Patients with cystic fibrosis (CF) were investigated for mucociliary clearance (with and without stimulation by terbutaline), clinical picture, ventilatory function and ultrastructure of cilia. The results were compared with those of patients with congenitally immotile cilia (immotile-cilia syndrome). Mucociliary clearance could be demonstrated in all the seven CF patients who succeeded in inhaling the test aerosol. Ciliary ultrastructure from a deceased CF patient was normal. Patients with the immotile cilia syndrome had no substantial clearance and defective cilia. The CF patients coughed more during the clearance measurements than any other group studied earlier, and their coughing was effective. One patient succeeded in avoiding coughing in both measurements and had faster clearance when he got terbutaline than when he got the vehicle. Although younger, the CF patients tended to be more obstructed in their lungs and more handicapped than the patients suffering from the immotile-cilia syndrome. The latter patients had more discomfort from rhinitis, sinusitis and otitis than had the CF patients. An impairment of the mucociliary transport rate is hence unlikely to be a primary pathogenic factor for the respiratory tract disease in CF patients.