Acute ST-segment elevation myocardial infarction occurs when the blood supply to a coronary artery is completely interrupted. The most common cause of the abrupt occlusion of a coronary artery is the rupture of a vulnerable atherosclerotic plaque. However, embolisms from any sources can be another cause of abrupt coronary obstruction. The authors report a case of acute ST-segment elevation myocardial infarction caused by a huge left atrial thrombus mimicking a myxoma.