The obstructive sleep apnea (OSA) syndrome has been considered to be a cause of both transient blood pressure elevations during sleep and sustained hypertension during the awake state. The purpose of this review was to examine critically the existing literature regarding (1) the blood pressure alterations associated with OSA, (2) causal mechanisms relating specific blood pressure alterations to OSA, and (3) potential consequences of the systemic circulatory abnormalities associated with OSA. Particular attention was directed at studies that assessed the prevalence of OSA in patients with hypertension and that examined the effects on blood pressure of treatment of OSA. We conclude that patients with OSA have abnormal sleep blood pressure patterns, manifested most frequently by apnea-associated blood pressure elevations. Confounding factors such as obesity and antihypertensive drug therapy, and conflicting evidence regarding changes in daytime blood pressure after therapy for OSA, make it premature to conclude that OSA and daytime hypertension are directly associated. Circumstantial evidence suggests that the blood pressure alterations that occur during sleep could contribute to the high cardiovascular morbidity in patients with OSA. Further research into the relationship between OSA and hypertension should improve the future care of patients with these conditions and enhance our understanding of cardiopulmonary pathophysiology.