Progressive alterations in the in vitro transport properties of airway secretions according to the severity and duration of bronchitis have been previously reported, but the potential in vivo dysfunction of bronchial clearance during different phases of bronchitis has not yet been studied. We measured bronchial mucociliary clearance (CMC) and clearance by cough (CTx) using a radioactive aerosol technique (resin particles 7.4 micron in diameter, tagged with technetium-99 m) in patients with recurrent bronchitis (group I, n = 5) and in chronic bronchitic patients, either at an early (group II, n = 13) or advanced (group III, n = 17) stage of their disease. The results were compared with those obtained from a control group (group 0, n = 10) and the respective efficiencies of the mucociliary system and cough were analysed with reference to respiratory function. The initial deposition of the aerosol in central airways is similar in all the patients and healthy subjects. In comparison with group 0, the mucociliary clearance is only significantly reduced in groups II and III (p less than 0.02). In healthy subjects, the clearance by cough is very weak, whereas in bronchitic patients it can be roughly equal to or greater than the CMC. However, the cough is less efficient in group III than in group II (p less than 0.05). A reduced efficiency of the clearance by cough occurs together with abnormalities in the gas exchange.