An echocardiography detected a large aneurysm, 10 x 15 cm in size, in the posterior wall of left ventricle in a 21-year-old male. This aneurysm could not be detected by either left or right ventriculogram. The aneurysm was opened during cardiac arrest with extracorporeal circulation, and red and white thrombi were found in it. Also the anurysm was communicated to the right atrium through a 1 x 1 cm hole, not to the left ventricle. For the complete resection of the anurysm, it was necessary to resect the left circumflex coronary artery running along the surface. The posterior wall movement in angiogram one month after surgery showed no change. Morphologically and histologically this case was similar to "Submitral annual left vetricular aneurysm" reported by Abrahams et al. 1962. It was a rare case because the anurysm was communicated to the right atrium.