The effect of manual percussion of the thorax in nine patients with stable chronic airflow obstruction and excessive tracheobronchial secretion has been studied. Tracheobronchial clearance was measured over 50 minutes on three different days. On the first day manual percussion was applied for 10 minutes. In the period when percussion was applied the mucus clearance was slightly but significantly greater than in the periods when no percussion was applied. On the second day manual percussion was applied in combination with postural drainage, coughing, and breathing exercises for 20 minutes. This resulted in a much greater clearance than on the first day. On the third day postural drainage, coughing, and breathing exercises, but no manual percussion, were carried out for 20 minutes. There was no significant difference between the clearance of days 2 and 3. From this study it is apparent that manual percussion is a relatively ineffective procedure in patients with stable chronic airflow obstruction, but may be useful when the patient is not able to cough and cannot assume the appropriate position for postural drainage.