Cardiopulmonary failure resulting from progression of obstructive sleep apnea (OSA) is treated with endotracheal intubation and mechanical ventilation. This study was conducted to determine whether the use of nasal continuous positive airway pressure (CPAP) would rapidly reverse changes in mental status and hypercapnic acidosis in such patients with decompensated hypercapnic respiratory failure resulting from OSA. Six morbidly obese patients (mean weight, 159 +/- 19 kg) were treated with nasal CPAP and supplemental oxygen. Within 24 h of this treatment, there was a significant increase in pH, from a baseline mean of 7.23 +/- 0.03 to 7.35 +/- 0.01 (p < 0.01), and the mean PaCO2 fell from 80 +/- 4 mm Hg to 64 +/- 4 mm Hg (p < 0.01). In addition, there was a dramatic improvement in mental status within 24 h of therapy with nasal CPAP. None of the patients required intubation and mechanical ventilation. There were no complications attributable to the CPAP delivered by nasal mask. We conclude that CPAP delivered by nasal mask can be safe and effective in rapidly reversing changes in mental status and hypercapnic acidosis in this group of patients with severe respiratory failure, and nasal CPAP obviates the need for endotracheal intubation and mechanical ventilation.