The aim of this study was to assess the effect of nasal continuous positive airway pressure (CPAP) breathing on patients with COPD when they are in respiratory failure during acute episodes of the disease. We studied 15 male patients with COPD who were in acute, unstable condition, without mechanical ventilatory support, before and during 4 h of nasal CPAP. The respiratory rate decreased (initial, 30 +/- 6; final, 25 +/- 6; p < 0.001), as did the subjective sensation of dyspnea (initial Borg index, 3 +/- 3; final, 2 +/- 2; p < 0.05). A significant decrease in the average PaCO2 (before, 73 +/- 10 mm Hg; after, 61 +/- 9 mm Hg; p < 0.001) and a significant increase in the PaO2 (before, 57 +/- 13 mm Hg; after 64 +/- 14 mm Hg; p < 0.05) were also observed. Nasal CPAP was generally well tolerated. According to these results, nasal CPAP should be considered as a possibility for treatment in patients with COPD who are in acute respiratory failure, in order to avoid mechanical ventilation.