The repetitive respiratory events that characterize obstructive sleep apnea (OSA) are each followed by abrupt increases in heart rate and in pulmonary and systemic artery pressure and by sudden decreases in right and left ventricular stroke volume. The changes in systemic pressure may be profound, with patients who are normotensive while awake having systolic pressures approaching 300 mm Hg after apnea termination. Because of these dramatic hemodynamic oscillations during sleep, many clinicians and investigators have postulated a connection between sleep-disordered breathing and cardiovascular morbidity and even mortality. This review critically examines the evidence for such a causal relationship. We begin, however, by reviewing the normal hemodynamic changes that occur during sleep. We then describe the acute hemodynamic events associated with OSA. Finally, we summarize the evidence for and against a causal connection between sleep apnea and cardiovascular morbidity.