We studied 138 patients (69 men and 69 women) with rupture of the left ventricular (LV) free wall during acute myocardial infarction (AMI) (rupture group) and compared clinical and necropsy findings in them with 50 patients who died during their first AMI without rupture (non-rupture group). The frequency of systemic hypertension (55%-vs.-52%), angina pectoris (13%-vs.-22%) and congestive heart failure (O%-vs.-O%) before the fatal AMI was similar for both rupture and non-rupture groups. Mean heart weights for men (479 g-vs.-526 g) and women (399 g-vs.-432 g) with and without rupture also were insignificantly different. LV scar before the infarct which ruptured was present in 18 patients (13%); previous necropsy studies of fatal AMI without rupture have indicated that 50% have LV scars. The rupture group had a significantly more frequent (P less than .01) lateral wall location of the infarct (12%-vs.-2%). The number of 3 major (right, left anterior descending and left circumflex) epicardial coronary arteries narrowed at some point greater than 75% in cross-sectional area by atherosclerotic plaque was significantly lower (P less than .Ol in the rupture group (39%-vs.-58%). The percent of these 3 arteries totally occluded or nearly so (less than 95% in cross- sectional area) by plaque also was significantly less (P less than .OOl) in the rupture group (24 of 198 arteries (12%)-vs.-38 of 144 arteries (26%)). Analysis of each 5-mm long segment of these arteries in each group disclosed that the rupture group had significantly less narrowing than the non-rupture group.