Delayed rupture of myocardial infarcts, more than 10 days after infarction, is only rarely observed at autopsy. Of 1,814 hearts examined after postmortem arteriography from autopsy subjects at the Johns Hopkins Hospital, 704 had 1,140 infarcts. Three (0.2 percent) infarcts were complicated by subepicardial aneurysms, and two of these had ruptured. The infarcts were 21 to 60 days old and had an abrupt dissection of the inferior left ventricular myocardium with a narrow fibrous lined channel comprising the mouth and neck of the aneurysm. The components of the aneurysm wall differed in all three cases. In two, epicardium prevented acute ventricular rupture. In the third case, myocardial fibers were present in the fibrous wall proximally. Adherent parietal pericardium was absent. Previous authors have required the lack of myocardium within the wall to define those lesions, often called pseudoaneurysms. Aneurysms with myocardium have been labeled true aneurysms or pseudo-false aneurysms. A more unifying concept of this lesion is proposed. The unique constellation of features consisting of an abrupt interruption of the myocardium, a narrow neck, and a propensity to rupture spontaneously distinguishes the subepicardial aneurysm regardless of its wall's components. The ability to surgically correct the potentially lethal subepicardial aneurysms necessitates a more accurate categorization of and familiarity with the lesion.