Coronary function versus anatomy, flow versus stenosis: which optimizes coronary artery disease (CAD) management? In patients, coronary flow is poorly related to stenosis severity, and revascularization fails to improve mortality over medical treatment in randomized trials. Yet percutaneous intervention (PCI) guided by fractional flow reserve reduces coronary events more than PCI guided by arteriographic stenosis. These paradoxes are explained by the poor relation between coronary flow reserve (CFR) and stenosis severity due to diffuse CAD, with surprising clinical implications. Should the concept of anatomically "critical" coronary stenosis be replaced by the concept of "critical" CFR reduction for managing CAD?