Thirty-eight patients (24 men and 14 women) with an acquired ventricular septal defect during acute myocardial infarction (AMI) (rupture group) were studied and their clinical and necropsy findings were compared with 50 patients who died during their first AMI without rupture (nonrupture group). The frequency of systemic hypertension (54 vs 52%), angina pectoris (28 vs 22%) and congestive heart failure (5 vs 0%) before the fatal AMI was similar for both rupture and nonrupture groups. Mean heart weights for men (498 vs 526 g) and women (397 vs 432 g) with and without septal rupture also were insignificantly different. Whereas previous studies of fatal AMI cases have shown that 50% of cases of fatal AMI without rupture have left ventricular scars, only 4 (10%) of the rupture cases had a left ventricular scar before the infarct that ruptured. The rupture group had a significantly more frequent (p less than 0.01) posterior location of the infarcts (74 vs 40%) and, therefore, a higher frequency of associated right ventricular infarcts 50 vs 18%). 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 of atherosclerotic plaque was the same in both groups. The percent of these 3 arteries totally occluded or nearly so (greater than 95% in cross-sectional area) by plaque was significantly less (p less than 0.001) in the rupture group compared with the nonrupture group (9 of 99 arteries [9%] vs 38 of 144 arteries [26%]).(ABSTRACT TRUNCATED AT 400 WORDS)