A 71-year-old woman with a history of previous myocardial infarction was transferred to our hospital for evaluation of chest pain and ventricular tachycardia. On admission, a loud mid-systolic ejection murmur accompanied by a thrill was found at the left sternal border in the third intercostal space, and it was significantly accentuated in the post-extrasystolic beat. Abnormal Q waves and ST elevations were noted in leads I, aVL and V5,6 on electrocardiograms. Echocardiograms, confirmed a septal-to-apical aneurysm, and a thin interventricular septum (IVS) with paradoxical motion. Right ventricular (RV) catheterization showed a pressure gradient of 21 mmHg between the outflow tract (RVOT) and the apex, and a mid-systolic ejection murmur was recorded in the RVOT on an intracardiac phonocardiogram. Coronary arteriograms revealed total occlusion of the left anterior descending artery in its proximal portion, and a 90% stenosis of the circumflex artery. A left ventriculogram demonstrated a septal-to-apical aneurysm with a markedly reduced ejection fraction of 0.16. A right ventriculogram showed obstruction to RVOT caused by systolic ballooning of the IVS. In this patient, the mid-systolic ejection murmur was probably caused by the obstruction of the outflow tract secondary to septal aneurysm following old myocardial infarction.