Cystic fibrosis (CF) is a deadly hereditary disease that produces an abnormally thick, viscous and abundant secretion in the respiratory tract. This secretion in turn leads to the development of recurrent respiratory infections and irreversible lung damage. We have studied nasal mucociliary transport by means of an isotopic technique in 12 patients with CF and in 12 healthy volunteers. Nasal mucociliary transport was repeated at 12-18 months in the patients. In five randomly selected patients ciliary ultrastructure was studied. The velocity of nasal mucociliary transport was significantly slower than in healthy persons (P < 0.001) and no significant differences were observed in both studies (P < 0.05). No significant differences were either observed in the CF group between the homo- and heterozygotes (P < 0.5), or in those six patients infected by Pseudomonas aeruginosa (P < 0.05). Ciliary ultrastructure was normal in one patient. In another patient the sample showed no cilia, while the remaining three exhibited changes similar to those observed in chronic respiratory infections: supernumerary peripheral tubules, ciliary disorientation and ciliary complexes.