The vascular beds of 42 human hearts were examined to determine the underlying morpho-functional arrangement of the coronary arteries. The hearts were obtained at autopsy from cases where the immediate cause of death was not related to cardiovascular disease. It was found that while the course and size of individual vessels vary considerably from heart to heart, there is an underlying pattern in the functional arrangement of the vessels. On the basis of this pattern, a new functional classification of coronary arteries is proposed, together with a related concept of intrinsic "zones" of the myocardium. It is concluded that a deficiency in coronary blood supply in ischemic heart disease would be more accurately described in terms of the affected zones of the myocardium rather than the affected vessels. It is the effect of stenosis, not the stenosis per se, which must be assessed and documented ultimately. A chart is proposed to facilitate that documentation.