The effects of postural drainage, exercise, and cough on mucus clearance were compared in 8 patients with chronic bronchitis. A bolus of 99mTc albumin aerosol was inhaled at a high inspiratory flow rate to enhance proximal deposition. Retention of deposited aerosol in the lung, as a function of time, was quantified using a gamma camera and subsequent computer analysis. Coughing greatly accelerated total lung (P less than 0.005) and peripheral (P less than 0.005) mucus clearance. Exercise resulted in much smaller changes than did cough, but significantly increased total lung clearance (P less than 0.005). Postural drainage in which coughing was prohibited did not alter clearance. These results have therapeutic implications and stress the importance of controlling cough when evaluating therapeutic interventions by these techniques.