The status of the native coronary arteries at necropsy in the vicinity of the coronary anastomoses of saphenous vein aortocoronary bypass grafts in 20 patients with severe coronary heart disease is presented. Of the 37 graft systems (graft plus coronary artery into which graft inserted) analyzed, the lumina of 44% of the native coronary arteries within the first 2 cm distal to the anastomoses were greater than 75% narrowed in cross-sectional area by atherosclerotic plaques, and the native coronary artery at the site of the anastomosis was greater than 50% narrowed in cross-sectional area already by atheroclerotic plaque in 25% of the graft systems. The mean coronary arterial size distal to the site of the coronary graft anastomosis, even after correction for heart weight, was greater in the 13 men than in the seven women. The residual luminal areas squared per gram of heart weight, however, were similar in both men and women. These results suggest that 1) relative coronary vessel size is greater in men than women; 2) the luminal area squared per gram myocardial mass (a relative estimation of flow) is the same in the two groups of patients; and 3) less atherosclerotic plaque is necessary in women then in men to produce similar limitation to coronary flow. Thus, vessel size alone cannot account for the higher reported frequency of unsuccessful aortocoronary bypass procedures in women.