Obstructive sleep apnea syndrome (OSAS), due to upper airway collapse, is frequent but still underestimated. The dose-response relation between OSAS and hypertension (HTN) is now well established. Logically, therefore, blood pressure must be tested in every apneic patient, if necessary by ambulatory blood pressure monitoring. Multiple mechanisms explain this relation, most importantly the increase in sympathetic activity during apnea episodes. OSAS-related hypertension has several characteristics: it is highly prevalent, predominantly diastolic and nocturnal, and frequently affects non-dippers; and the HTN tends to be resistant to treatment. OSAS promotes the formation of arterial lesions (parietal thickening of the carotid artery, increased aortic stiffness, and endothelial dysfunction); the more severe the OSAS, the more severe the lesions. The beneficial effects on blood pressure of continuous positive airway pressure (CPAP), the benchmark treatment for OSAS, are still debated but appear to be significant for untreated or refractory hypertension, for severe OSAS, and when CPAP compliance is good. It also seems promising for the reduction of arterial lesions linked to OSAS.