An interseptal false aneurysm of the left ventricle due to the dissection of the septum in a patient with acute inferior myocardial infarction is described. The aneurysm was demonstrated as a cystic bulge of the left ventricular cavity into the inferoposterior interventricular septum with a small orifice from the left ventricle without any protrusion or rupture into the right ventricular cavity. Two-dimensional echocardiography, magnetic resonance imaging, and dynamic computed tomography were the most useful and reliable noninvasive diagnostic modalities. Repeated examinations demonstrated a significant reduction of the aneurysm in six months.