Study Objectives: Investigate the efficacy of a novel nasal expiratory positive airway pressure (EPAP) device as a treatment for obstructive sleep apnea (OSA). Design: A prospective, multicenter, sham-controlled, parallel-group, randomized, double-blind clinical trial. Setting: 19 sites including both academic and private sleep disorder centers Patients: Obstructive sleep apnea with a pre-study AHI = 10/hour Interventions: Treatment with a nasal EPAP device (N = 127) or similar appearing sham device (N = 123) for 3 months. Polysomnography (PSG) was performed on 2 non-consecutive nights (random order: device-on, device-off) at week 1 and after 3 months of treatment. Analysis of an intention to treat group (ITT) (patients completing week 1 PSGs) (EPAP N = 119, sham N = 110) was performed. Measurements and Results: At week 1, the median AHI value (device-on versus device-off) was significantly lower with EPAP (5.0 versus 13.8 events/h, P < 0.0001) but not sham (11.6 versus 11.1 events/h, P = NS); the decrease in the AHI (median) was greater (-52.7% vs. -7.3%, P < 0.0001) for the ITT group. At month 3, the percentage decrease in the AHI was 42.7% (EPAP) and 10.1% (sham), P < 0.0001. Over 3 months of EPAP treatment the Epworth Sleepiness Scale decreased (9.9 +/- 4.7 to 7.2 +/- 4.2, P < 0.0001), and the median percentage of reported nights used (entire night) was 88.2%. Conclusions: The nasal EPAP device significantly reduced the AHI and improved subjective daytime sleepiness compared to the sham treatment in patients with mild to severe OSA with excellent adherence.