Nasal ciliary function and mucociliary clearance were studied in patients with cystic fibrosis and in three control groups. Ciliary beat frequency and nasal clearance time were measured in groups of 10 subjects with cystic fibrosis, sinusitis and bronchiectasis and age and sex-matched control subjects. Ciliary beat frequency was also measured in normal subjects matched as bronchiectasis controls. Cystic fibrosis patients and their controls, patients with sinusitis, and the bronchiectasis controls did not differ in ciliary beat frequency, but it was slower in the patients with bronchiectasis (p less than 0.05). Nasal mucociliary clearance in cystic fibrosis and bronchiectasis was slower than in the cystic fibrosis controls (p less than 0.001) and in patients with sinusitis (p less than 0.01). The finding of a normal beat frequency in cystic fibrosis cilia studied in vitro together with abnormal nasal mucociliary clearance measured in vivo in the same patients suggests the existence of an abnormality of mucus in vivo. The innate function of cystic fibrosis cilia, as measured in vitro by beat frequency, is normal.