The interventricular septum is one of the three main sites at which the myocardium can rupture. The features of the interventricular septal rupture that occurred in a 72-year-old woman are characteristic of interventricular septal ruptures in general: (1) they occur most commonly in elderly women; (2) the most common site is the mid-portion of an acute, transmural anteroseptal apical infarct; (3) they are also most common during the patient's first heart attack; (4) the clinical diagnosis of acute myocardial infarct is confirmed by both ECG and by serum enzyme levels; (5) the usual time of the rupture is 3-10 days after the onset of the infarction (it occurred after 3 days in our patient); (6) a new cardiac murmur usually is heard and the patient frequently goes into shock; (7) the diagnosis can be confirmed by a step-up in pO2 levels from right atrium to right ventricle; (8) the usual cause is severe old coronary atherosclerosis with a recent thrombotic occlusion as the final precipitating event.