The main objective of the study is to compare the decrease of mean arterial blood pressure over 24 hours in patients having obstructive sleep apnea syndrome and weak or moderate hypertension, treated by Valsartan 160mg per day versus continuous Positive Airway Pressure.

It is now well known that cardiovascular risks are increased in patients with obstructive sleep apnea syndrome (OSAS). Also a link has already been demonstrated between OSAS and hypertension.

Nowadays, the most efficient treatment of the OSAS is the continuous Positive Airway Pressure (cPAP). Several studies have also shown that cPAP could reduce arterial blood pressure of about 3,3 mmHg in OSAS patients. Unfortunately, 40% of the patients either refuse being treated by cPAP or give up treatment. In parallel, alternative medications are proposed to patients with moderate OSAS and hypertension, in order to decrease their cardiovascular risks. A study led on rats has shown the superiority of the beta adrenergic blocking agents in the decrease of hypertension in OSAS patients.

Our study has also the objective to compare the effects of Valsartan and cPAP on hypertension in OSAS patients. Those one will be randomized either in the group "treatment by Valsartan and then by cPAP" or in the group "treatment by cPAP and then Valsartan" (cross-over study).

Inclusion Criteria:

• male/female over than 18 years old
• patient with an obstructive sleep apnea (apnea-hypopnea index > or equal to 15)
• patient with weak or moderate hypertension (140 <= SBP < 180 mmHg and 90 <= DBP < 110 mmHg). Patient with antihypertensive monotherapy must stop their treatment and respect a washout period of at least 4 weeks before the screening visit. For patient treated by antialdosterone, the washout period will be of at least 8 weeks.
• negative pregnancy test
• ambulatory patient
• patient who have signed the informed consent form
• patient affiliated to social security

Exclusion Criteria:

• pregnant or nursing woman
• woman who refuses to use contraceptive method
• acute hepatic failure, biliary cirrhosis, cholestasis
• clearance of Cockcroft < 30 ml/min/1.73m2
• kaliemia >= 5.5 mmol/l
• acute hypertension (SBP>= 180 mmHg and/or DBP >= 110 mmHg)
• acute daytime sleepiness (Epworth rating scale > 15)
• patient with a profession that is inconsistent with the continuous positive airway pressure (CPAP) treatment
• known cardiovascular pathologies
• contraindication to CPAP
• allergy to Valsartan
• patient treated with lithium
• patient treated with drug(s) acting on arterial blood pressure
• patient on tutelle or curatelle
• patient kept in detention, major protected by the law, hospitalised person
• patient currently participating in another clinical trial