RATIONALE: Continuous positive airway pressure (CPAP) is the current treatment for patients with symptomatic obstructive sleep apnea (OSA). Its use for all subjects with sleep-disordered breathing, regardless of daytime symptoms, is unclear. OBJECTIVES: This multicenter controlled trial assesses the effects of 1 year of CPAP treatment on blood pressure (BP) in nonsymptomatic, hypertensive patients with OSA. METHODS: We evaluated 359 patients with OSA. Inclusion criteria consisted of an apnea-hypopnea index (AHI) greater than 19 hour(-1), an Epworth Sleepiness Scale score less than 11, and one of the following: under antihypertensive treatment or systolic blood pressure greater than 140 or diastolic blood pressure greater than 90 mm Hg. Patients were randomized to CPAP (n = 178) or to conservative treatment (n = 181). BP was evaluated at baseline and at 3, 6, and 12 months of follow-up. MEASUREMENTS AND MAIN RESULTS: Mean (SD) values were as follows: age, 56 +/- 10 years; body mass index (BMI), 32 +/- 5 kg x m(-2); AHI, 45 +/- 20 hour(-1); and Epworth Sleepiness Scale score, 7 +/- 3. After adjusting for follow-up time, baseline blood pressure values, AHI, time with arterial oxygen saturation less than 90%, and BMI, together with the change in BMI at follow-up, CPAP treatment decreased systolic blood pressure by 1.89 mm Hg (95% confidence interval: -3.90, 0.11 mm Hg; P = 0.0654), and diastolic blood pressure by 2.19 mm Hg (95% confidence interval: -3.46, -0.93 mm Hg; P = 0.0008). The most significant reduction in BP was in patients who used CPAP for more than 5.6 hours per night. CPAP compliance was related to AHI and the decrease in Epworth Sleepiness Scale score. CONCLUSIONS: In nonsleepy hypertensive patients with OSA, CPAP treatment for 1 year is associated with a small decrease in BP. This effect is evident only in patients who use CPAP for more than 5.6 hours per night. Clinical trial registered with www.clinicaltrials.gov (NCT00127348).