Acute myocardial infarction is predominantly caused by coronary artery atherosclerotic plaque rupture and subsequent occlusive thrombus formation. The recognition of less common causes of acute myocardial infarction is important because they may require a different treatment strategy. We report a patient with acute myocardial infarction without any angiographic evidence of coronary atherosclerosis and a left atrial mass detected on echocardiography. Therefore, coronary embolism from intracardiac thrombus or tumor was suspected. No additional manifestations of a potential tumor were found on thoracic, abdominal and cranial computed tomography. During subsequent cardiac surgery, a large tumor could be in toto resected and was diagnosed as a highly malignant leiomyosarcoma on histopathological evaluation.