The effect of continuous positive airway pressure (CPAP) applied via a mask covering the nose and mouth (oral-nasal CPAP = ONCPAP) on obstructive sleep apnea (OSA) was studied in ten male patients with a mean (+/- SD) age of 48.1 +/- 11.1 years who could not tolerate nasal CPAP (NCPAP) due to nasal congestion. Using ONCPAP at pressures of 11.0 +/- 4.5 cm H2O, the apnea+hypopnea index was reduced from 58.3 +/- 22.3 (baseline night) to 5.2 +/- 1.6 events per hour (ONCPAP night) (p < 0.001). Five of these patients were studied on a subsequent night with a dual chamber mask allowing separate measurement of nasal and oral flow. All patients had combined oral and nasal flow at times during the night, but the fraction of time spent with this breathing pattern was lower during sleep than wakefulness. In a separate study, we compared the effects of a therapeutic level of CPAP pressure (12.8 +/- 2.5 cm H2O) applied through a nasal mask (NCPAP) and ONCPAP in a different group of patients (mean age 60 +/- 14.6 years) with moderate to severe OSA using NCPAP on a long-term basis. The apnea-hypopnea indexes on NCPAP nights (7.2 +/- 3.5) and ONCPAP nights (7.6 +/- 4.9 events per hour of sleep) were very similar. We conclude that ONCPAP may be a reasonable treatment alternative in patients who cannot tolerate NCPAP due to nasal congestion and that the pressure required to maintain upper airway patency may be similar to the level required using NCPAP.