Background: Continuous positive airway pressure (CPAP) is the first line therapy for obstructive sleep apnea syndrome (OSAS). Two positive airway pressure (PAP) modalities can be used: Fixed pressure, in which the effective pressure is set and kept constant all night, and auto CPAP devices where the positive pressure changes during the night depend on patient's requirements.

Hypothesis: Pressure variations associated with autoCPAP functioning can lead to sleep fragmentation and alterations in sleep structure. This can limit blood pressure dipping during sleep and then impact clinical blood pressure reduction.

Main objective: To compare in a randomized controlled trial 4 months clinical blood pressure (BP) evolution depending on CPAP mode: fixed pressure versus autoCPAP. Secondary outcomes: Evolution of arterial stiffness, biological parameters, quality of life and symptoms.

Methods: Patients will be randomised depending on CPAP mode. Baseline and 4 months evaluation will include: 24-h ambulatory blood pressure monitoring, clinical BP measurements and carotid-to-femoral pulse wave velocity (PWV). Patients will also complete quality of life and symptoms questionnaires.

2 interim analysis will be carried out when 150 and 220 patients respectively will have completed the study. The Peto's method will be used to correct the p-values.

Inclusion Criteria:

• age between 18 and 80 years
• patients who gave written informed consent
• patients who subscribed a social insurance
• SAOS patients who need CPAP treatment.

Exclusion Criteria:

• cardiac failure known and treated
• central Apnea syndrome
• patients who stopped CPAP treatment in the previous year
• pregnant women
• patients under guardianship
• imprisoned patients, patients in hospital
• patients included in an other clinical study