A qualitative and quantitative analysis of the amount of myocardial scarring and the degree and extent of coronary arterial narrowing by atherosclerotic plaque in the entire lengths of each of the four major epicardial coronary arteries is described in 18 necropsy patients with healed transmural myocardial infarcts and death from a non-cardiac condition. An average of 30 per cent of the basal half and 38 per cent of the apical half of the left ventricular wall was scarred. The nine patients with clinical evidence of previous acute myocardial infarction tended to have larger left ventricular scars than the nine patients without such evidence but the difference was not significant. An average of 26 cm (51 5 mm segments) of coronary artery were examined from each patient ad 25 cm (49 5 mm segments) from each of 19 control subjects. Of 924 segments examined in the 18 patients, 292 (32%) were 76 to 100 per cent narrowed in cross-sectional area (controls = 5); 321 (35%) were 51 to 75 per cent narrowed (controls = 34%); 210 (23%) were 26 to 50 per cent narrowed (controls = 44%), and 101 (11%) were 0 to 25 per cent narrowed (controls = 17%). The extent of severe narrowing of 75 per cent or more was similar (25%) in the left anterior descending and left circumflex coronary arteries; the right was the most severely narrowed artery and the left main was not severely narrowed in any patient. Excluding, then, the left main artery, the amount of severe narrowing in the proximal and distal halves of the other three vessels was similar. The amount of severe narrowing was not related to the age at death or to heart weight, but was greater in patients with hypertension or with a history of acute myocardial infarction.