The effects of a 30-minute period of chest percussion and postural drainage were compared to a sham treatment (infrared lamp) in 35 patients with stable chronic bronchitis and to a period of directed coughing in 11 of these same patients. There were no differences in subjective responses or arterial blood gas levels following therapy. Spirometric studies showed small improvements over baseline values following either treatment but no difference between active and sham treatments. The volume of sputum expectorated during percussion and drainage was significantly greater than during the infrared warming (5.5 vs 1.4 ml) or during the directed coughing (9.0 vs 3.5 ml). Although chest percussion and postural drainage are effective in augmenting the volume of expectorated sputum, no significant alternations in air flow or gas exchange after two hours were demonstrated.