BACKGROUND: Dynamic hyperinflation, caused by expiratory flow limitation, markedly increases resting end-expiratory lung volume (functional residual capacity) in many COPD patients. OBJECTIVE: To determine the impact and duration of impact of CPAP on hyperinflation and airway resistance in patients with stable COPD. METHODS: In a case series, 21 patients underwent CPAP at 8 cm H 2 O for 15 min, then whole-body plethysmography immediately after, and at 15 and 30 min after CPAP. RESULTS: The cohort's mean ؎ SD age was 70 ؎ 9 y, and the mean FEV 1 was 41 ؎ 8% of predicted. Residual volume, functional residual capacity, total lung capacity, the ratio of residual volume to total lung capacity, and airway resistance decreased after CPAP and did not significantly change at 15 min (P < .001), but returned to baseline at 30 min. CONCLUSIONS: In patients with severe to very severe stable COPD, CPAP reduces lung volumes and airway resistance for 15 min, but the lung volumes return to baseline by 30 min.