Two respiratory physiotherapy methods were evaluated by analyzing changes in the transcutaneous partial pressure of CO2 (tcPCO2) in 15 patients with respiratory insufficiency. All were receiving continuous oxygen therapy and had CO2 retention. The physiotherapy methods compared were thoracic compression (TC) and positive expiratory pressure (PEP). The mean decrease (+/- SD) in tcPCO2 was the same with both methods, 0.6 +/- 0.4 kPa, but there were interindividual differences. This study indicates that both TC and PEP effectively decrease tcPCO2, although only temporarily in many cases. Continuous tcPCO2 monitoring is a useful clinical method for determining and teaching optimal respiratory therapy.