<qa>  What is the primary endpoint of this study?Exercise training has long been recommended as a means of improving cardiac function and reducing morbidity and mortality in patients with coronary artery disease CAD. One mechanism of benefit may be through improved endothelial function and enhanced nitric oxide bioactivity. Such an effect may augment blood flow to exercising skeletal muscle and to the myocardium. In this regard, studies in both animals and humans have shown that frequent exercise improves endothelial function as well as blood flow to ischemic myocardium. Exercise-induced improvement in endothelial function is believed to result from up-regulation of endothelial nitric oxide synthase eNOS transcription due to repetitive shear stress associated with exercise. However, not all patients show improved endothelial function or myocardial perfusion despite compliant participation in cardiac rehabilitation programs. We hypothesize that endothelial function before and following structured exercise programs may be determined by the presence and function of endothelial progenitor cells EPCs that are released from the bone marrow and circulate in the blood stream. Thus, patients who have low baseline or post-exercise EPC levels or function may have limited capacity to improve endothelial function, exercise duration, and myocardial perfusion and function following participation in cardiac rehabilitation programs with repetitive exercise. On the other hand, patients with higher levels of EPCs at baseline or following repetitive exercise, with preserved capacity of EPCs to form endothelial cells, may show improved endothelial function, exercise duration, and myocardial perfusion and function as a result of repetitive exercise. The primary endpoint of this study will be improvement in endothelial function as determined by flow-mediated dilator responsiveness of the brachial artery at 2-3 months of repetitive exercise in the program compared with baseline measurements. Secondary endpoints include treadmill exercise duration and magnetic resonance imaging MRI-determined regional cardiac perfusion by gadolinium distribution during pharmacologic stress at 2-3 months compared with baseline measurements. EPCs will be identified by characteristic cell surface markers and by colony-forming ability at specific time-points of the study, with correlations determined between EPCs at baseline or the increase in EPCs following repetitive exercise, and the primary and secondary endpoints. We propose that patients most likely, and least likely, to benefit from a cardiac rehabilitation program may be identified on the basis of circulating EPCs. 