In 32 necropsy patients who died within 30 days of an aortocoronary bypass operation performed for relief of angina pectoris, the lumens in 42 (95 percent) of 44 nonbypassed and in 52 (100 percent) or 52 bypassed arteries were narrowed 76 to 100 percent in cross-sectional area by atherosclerotic plaque. Of 616 five mm segments of the 44 nonbypassed arteries examined histologically, 292 (47 percent) were narrowed 76 to 100 percent in cross-sectional area by atherosclerotic plaque; of 728 segments examined in the 52 bypassed arteries, 375 (52 percent) were similarly narrowed. Thirty-two (73 percent) of the 44 nonbypassed coronary arteries (in 23 patients) had been judged to be narrowed 50 percent or less in diameter on preoperative coronary angiography, but at necropsy 31 (97 percent) of these arteries were narrowed 76 to 100 percent in cross-sectional area and the other artery was narrowed 51 to 75 percent. Thus, significant amounts of atherosclerotic plaque tend to be present at necropsy in all three major coronary systems of patients with angina pectoris who die early after an aortocoronary bypass operation.