Tracheobronchial clearance was studied twice in 16 patients with chronic obstructive bronchitis after inhalation of 6 microns (aerodynamic diameter) monodisperse Teflon particles labeled with 111In. At one exposure the particles were inhaled at an extremely slow flow, 0.05 L/s; at the other they were inhaled at a normal flow, 0.5 L/s. Theoretical calculations and experimental data in healthy subjects indicate particle deposition mainly in the smallest ciliated airways using 0.05 L/s, i.e., in the bronchiolar region, and an enhanced deposition in larger airways using 0.5 L/s. Lung retention was measured at 0, 24, 48 and 72 h. Clearance was significantly every 24 h for both exposures (p < .05). The fractions of retained particles were significantly larger for particles inhaled at 0.05 L/s compared to 0.5 L/s at all points of time (p < .001). Compared to healthy subjects, the retained fractions of deposited particles were larger in patients with bronchitis breathing at 0.05 L/s, but smaller with breathing at 0.5 L/s (p < .01). Significant relationships were found between lung retentions and airway resistance (Raw) at 0.5 L/s, r = -.68 (p < .01), but not at 0.05 L/s, and between lung retention at 24 h and weight of expectorated sputum at 0.05 L/s, r = -.50 (p < .05). There was, furthermore, an almost significant relationship between sputum volume and rate of tracheobronchial clearance between 0 and 24 h (in percentage of the total amount cleared during 72 h) at 0.05 L/s, r = .42 (p = .05). The results indicate that in patients with chronic bronchitis overall clearance of particles in small airways is incomplete, as compared to larger airways. An increased amount of mucus, however, seemed to improve clearance of peripherally deposited particles, possibly by making cough more effective in small airways.