This study aimed to investigate the effect of increased lung volume with positive end-expiratory pressure (PEEP) on respiratory resistance in patients with chronic obstructive pulmonary disease (COPD). Ten patients with COPD were mechanically ventilated for acute respiratory failure. PEEP was set at 0, 5, 10 and 15 cm H2O. Using the rapid airway occlusion technique, the total inspiratory resistance of the respiratory system was partitioned into interrupter (Rint,rs) and additional effective (deltaRrs) resistances. At each level of PEEP, at constant inflation flow, the inflation volume (deltaV) was varied from 0.2-1 L, and, at constant deltaV, the inflation flow was varied from 0.2-1.2 L x s(-1). The changes in end-expiratory lung volume (deltaEELV) induced by PEEP were also measured. The difference between the EELV and the relaxation volume of the respiratory system (deltaFRC) increased significantly with PEEP of 10 and 15 cm H2O as compared to a PEEP of 0, the increase being associated with a significant reduction of Rint,rs. By contrast, deltaRrs was independent of deltaFRC. At constant deltaV, Rint,rs fitted Rohrer's equation (Rint,rs = K1 + K2 x flow). While K2 significantly declined with AFRC, K1 did not change. At all levels of PEEP, deltaRrs was not influenced by deltaFRC. With increasing lung volume induced by positive end-expiratory pressure, the inspiratory airway resistance decreased, whereas the viscoelastic behaviour of the respiratory system, as reflected by additional effective resistance, did not change.