A technique is introduced to quantify the microvasculature of postmortem human hearts. The method combines coronary angiography with selective perfusion of the coronary arteries with differently labeled radioactive microspheres. The transmural distribution pattern of the microspheres (endocardium-epicardium ratios) reveals that rigor mortis is a factor that should be taken into account. In the majority of cases, however, noticeable effects of rigor mortis were absent. The results of the perfusion of 19 human hearts with 10 to 35 microns sized microspheres indicate that the distribution pattern of the microspheres is determined by arterioles with a minimal diameter of 35 microns. In a second series of 15 human hearts, a comparison in the pattern of distribution of microspheres was made between hearts with obstructive coronary artery disease (n = 9) and hearts without obstructive disease (n = 6). The results support the contention that the present technique allows us to quantify differences in collateral vascular supply. Hence, the method may be of use for the study of the microvascular bed in the zone at risk in hearts of patients who have died of myocardial infarction.