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%) of 44 nonbypassed and in 52 (100%) of 52 bypassed arteries were narrowed 76 to 100% in cross-sectional area by atherosclerotic plaque. Of 616 five mm segments of the 44 nonbypassed arteries examined histologically, 292 (47%) were narrowed 76 to 100% in cross-sectional area by atherosclerotic plaque; of 728 segments examined in the 52 bypassed arteries, 375 (52%) were similarly narrowed. Thirty-two (73%) of the 44 nonbypassed coronary arteries (in 23 patients) had been judged to be narrowed 50% or less in diameter on preoperative coronary angiography, but at necropsy 31 (97%) of these arteries were narrowed 76 to 100% in cross-sectional area and the other artery was narrowed 51 to 75%.