The effect of breathing with a positive expiratory pressure of 5 cm H2O was investigated in eight patients with COPD (mean [SD]FEV1 = 54  percent predicted). Specific work of breathing (Wsp) and myoelectrical activity of the following respiratory muscles were measured at rest: scalene muscle, parasternal muscle, and abdominal muscles. Minute ventilation (VE), end-tidal CO2 (FETCO2), physiologic dead space ventilation (VD/VT), oxygen uptake (VO2), and carbon dioxide output (VCO2) were measured at rest and during an incremental bicycle exercise test. Dyspnea sensation during exercise was quantified using the CR10 Borg-scale. All measurements were performed with and without positive expiratory pressure (PEP). During PEP breathing at rest mean (SEM) Wsp increased from 0.54 (0.13) J/L to 1.08 (0.10) J/L. The SEM VE decreased from 12.4 (1.0) L/min to 10.5 (1.1) L/min, and SEM VD/VT decreased from 0.39 (0.03) to 0.34 (0.03). There was a tendency for an increased phasic respiratory muscle activity during PEP breathing of all three muscles as compared with undisturbed breathing, but the changes were not statistically significant. During the exercise test with PEP, VE, VD/VT, VO2, and VCO2 were significantly lower, and FETCO2 was significantly higher as compared with the values obtained during the exercise test with undisturbed breathing. Dyspnea sensation during the exercise test with PEP, however, was higher than during the test with undisturbed breathing. The PEP breathing at rest may be useful in patients with COPD as it increases the efficiency of ventilation by reducing dead space ventilation. This beneficial effect also occurs during exercise, but here it is accompanied by increased dyspnea sensation.