Evidence suggests that untreated obstructive sleep apnea (OSA) is a significant health risk for the development of hypertension, cardiovascular disease, and stroke. OSA is independently associated with obesity, hypertension, and insulin resistance/diabetes mellitus, the three stroke risk factors of the metabolic syndrome. Studies in normal subjects and sleep apneic patients suggest that upper airway occlusion induces arousal from non-rapid eye movement (NREM) sleep once the level of inspiratory effort reaches a certain value, which varies among individuals. During apnea there are several stimuli that are well known to be able to induce arousal, including hypercapnia, hypoxia, and increased airway resistance. Cohort studies have shown OSA to be a risk factor for stroke. OSA is associated with a variety of stroke risk factors that may independently contribute to stroke risk. This suggests the potential for a cause-and-effect relationship between untreated OSA and stroke in some cases.