In patients with severe chronic obstructive pulmonary disease (COPD), lung emptying may be affected by flow limitation. We tested the hypothesis that the airway compression leading to flow limitation can be counteracted by controlling the expiratory flow. The effects of an external resistor on lung emptying were studied in six patients with COPD, who were mechanically ventilated whilst sedated and paralysed. Respiratory mechanics were obtained during ventilatory support with and without the resistor. Airway compression was assessed using the interruptor method. For the study, a turbulent resistor was applied with the highest resistance level that did not increase the end-expiratory lung volume. At this resistance level, external positive end-expiratory pressure (PEEP) was generated in all patients. As total PEEP levels remained unchanged at both settings during the controlled expiration, the levels of intrinsic PEEP were significantly decreased from 0.96+/-0.30 to 0.53+/-0.19 kPa (mean+/-SD). Comparison of the expiratory flow-volume curves at both settings revealed that, during the controlled expiration, the flows were significantly decreased during the first 40% of the expired volume and significantly increased during the last 60%. As the end-expiratory lung volumes remained unchanged during both settings, these increments in flow indicated a decrease in effective resistance. Airway compression was observed during unimpeded expirations in all patients using the interruptor method. During the application of the resistor, airway compression was no longer detectable. In patients with chronic obstructive pulmonary disease receiving ventilatory support, the application of an external resistor could decrease effective expiratory resistance by counteracting airway compression, without increments in end-expiratory lung volume.