Of 10 patients with fatal coronary heart disease undergoing coronary angiography 0 to 69 d (average, 21) before necropsy, the amount of narrowing in 61 coronary arteries observed angiographically (diameter reduction) during life by three angiographers was compared with that observed histologically (cross-sectional area) at necropsy. No overestimations of the degree of narrowing were made angiographically. Of 11 coronary arteries or their subdivisions narrowed 0 to 50% in cross-sectional area histologically, none were underestimated angiographically; of eight narrowed 51% to 75% histologically, seven had been underestimated, and of 42 narrowed 76% to 100% histologically, 17 were underestimated angiographically. The coronary atherosclerotic plaquing was diffuse (greater than 25% cross-sectional area narrowing) in 90% of 467 five-millimetre segments of coronary artery examined (24 cm per patient), and this diffuseness of the atherosclerotic process seems to be the major reason for angiographic underestimation of coronary narrowings.