Impulse oscillometry (IOS) is a forced oscillation technique that enables pulmonary functional studies to be performed without requiring strenuous maneuvers. IOS assesses different components of respiratory impedance. The aim of this study was to compare the inspiratory and expiratory IOS parameters in COPD patients. IOS and spirometry were performed in 15 COPD patients and 23 healthy subjects. Thereafter, COPD patients were treated with tiotropium and their pulmonary function was re-evaluated. In COPD patients, the variations in the IOS parameters were significantly larger during expiration than during inspiration. The improvement in R5-20 (the difference between the respiratory resistance at 5 and 20 Hz, which reflects the distal lung resistance) after tiotropium treatment was statistically detected only during inspiration (p=0.008), not during expiration (p=0.139). In conclusion, the expiratory IOS parameters varied more than the inspiratory parameters, particularly in COPD patients-possibly because of flow-limitation during expiration. Thus, the evaluation of IOS parameters may be more accurate during inspiration in COPD patients.