Regional compliance of the ascending aorta, aortic arch, and the descending aorta was measured in 70 normal subjects at varying ages, in 17 patients with coronary artery disease (10 coronary artery disease patients, 3 with syndrome X), and in 13 trained athletes using magnetic resonance imaging. Ascending aortic compliance was measured angiographically in 22 patients with documented coronary artery disease and in 11 patients with syndrome X. Magnetic resonance velocity mapping was used in six patients with documented coronary artery disease and in three patients with syndrome X to study two-dimensional velocity profiles in the proximal and mid-ascending aorta and to quantify both forward and reverse flow. The measurements were compared with earlier published measurements from 24 normal subjects. It was found that patients with ischemic heart disease or syndrome X had decreased or no measurable aortic compliance and that they had significantly reduced or abnormal ascending aortic reverse flow likely to cause reduced coronary artery flow. A new theory is advanced that decreased myocardial perfusion leading to ischemic heart disease has two sources: (1) insufficient blood flow into the coronary artery inlet due to abnormal aortic function and independent of coronary artery stenosis and (2) local coronary artery stenosis. Observations supporting the theory are presented.