The fundamental determinants of coronary blood flow include the vessel radius, pressure gradient, and the physical characteristics of the cellular components and fluid medium. Specifically, cellular rheology and plasma viscosity determine, to a significant degree, both macro- and microcirculatory blood flow, particularly when either is compromised by existing atherosclerotic narrowing or reperfusion injury. Among individuals with coronary heart disease, abnormalities in cellular rheology and plasma viscosity may be the best predictors of subsequent cardiac events. Therefore, efforts to limit morbidity and mortality may depend on a more in-depth understanding of these basic areas.