Fifty-three hearts with rupture of the ventricular septum complicating acute myocardial infarction (AMI) were studied. Thirty-three of the hearts were from men (average age 76 years) and 20 were from women (average age 73 years). The study showed 2 types of rupture of the ventricular septum: simple (28 patients) and complex (25 patients). Simple ruptures were direct through-and-through defects. Complex ruptures were associated with serpiginous dissection tracts remote from the primary site of tear of the ventricular septum. Specimens were classified as to the location of the underlying AMI and the level of the septum (apex to base) at which the rupture occurred. Twenty-nine hearts had an inferior AMI and 24 an anterior AMI. Complex ruptures occurred in 20 of the inferior AMIs (69%) and in 5 of the anterior AMIs (21%) (p less than 0.001). Ruptures that involved the inferobasal portion of the septum were much more likely to be complex (94%) than ruptures in all other locations (27%, p less than 0.001). Significant 3-vessel obstructive coronary arterial atherosclerosis was present in 48 hearts. Rupture of a second structure in addition to the ventricular septum was observed in 11 hearts (left ventricular free wall in 9 cases and papillary muscle in 2). The interval from the onset of the AMI to rupture of the septum could be estimated in 22 patients and averaged 4 days (median 2.5 days). Complete heart block reportedly occurred in 6 patients during hospitalization.