Myocardial hibernation is a state of persistently impaired left ventricular function in patients with coronary artery disease that was thought to be caused by a chronic reduction in resting myocardial blood flow in a segment subtended by a diseased coronary artery. However, recent studies using positron emission tomography have demonstrated that absolute myocardial blood flow (ml/min/g) to hibernating myocardium is within normal limits in most patients. If resting flow is not reduced, one must therefore suspect an alternative "trigger" for hibernation that is still a consequence of coronary artery disease and ischemia. We suspect that hibernating myocardium may be the result of repetitive myocardial stunning. Myocardial stunning is the reversible contractile dysfunction occurring after a period of myocardial ischemia that persists for a period of time despite the return of blood flow to normal. Myocardial stunning has been demonstrated in humans in the setting of thrombolysis, coronary angioplasty, coronary artery bypass surgery, and coronary artery spasm. Furthermore, stunning has been demonstrated after exercise in patients with coronary artery disease, and recent studies have provided evidence that repetitive episodes of exercise-induced ischemia can lead to cumulative and prolonged left ventricular dysfunction.