1 INTRODUCTION
Left atrial (LA) thrombus is associated with atrial fibrillation (AF) and mitral valve disease in most cases. Although other conditions such as low cardiac function and coagulation abnormalities can cause LA thrombi, LA thrombus without atrial fibrillation or mitral valve disease is very rare. Most LA thrombi form within the left atrial appendage, but when thrombi form in the left atrium, distinguishing the thrombi from the LA tumors is often difficult. We report a case of a man with no history of AF or mitral valve disease who underwent LA mass resection for suspected LA tumor, which turned out to be a large thrombus after the surgery.