Summary: Obstructive sleep apnea syndrome (OSAS) is a disorder characterized by repetitive episodes of apnea or hypopnea during sleep. Apnea and hypopnea cause temporary elevations in blood pressure associated with blood oxygen desaturation, arousal, and sympathetic activation and may cause elevated blood pressure during the daytime. Numerous studies have shown that OSAS was associated with an increased cardiovascular morbidity and mortality. In this review, we describe the normal hemodynamic changes that occur during sleep, the acute hemodynamic events associated with OSAS and finally we summarize the evidence for causal connection between sleep apnea and cardiovascular disease.