Cough is a host defence mechanism of the lungs. Theoretical considerations indicate that the efficacy of cough in clearing secretions is limited to the first few airway generations of the human lung. Studies utilizing radioaerosols as markers of mucus support the theoretical predictions and show that cough is effective in the proximal airways only in the presence of excess secretions. Limited studies on the efficacy of cough on regional clearance of secretions have been equivocal regarding its effect in the "peripheral" regions of the lungs. Persistent coughing can result in temporary slowing or even cessation of lung mucociliary clearance at the flow-limiting segments of the lung. The forced expiration technique, which is not accompanied by the compressive phase associated with cough, is more effective than coughing in clearing excess secretions from the lungs.