Left atrial appendage closure is widely performed in cardiac surgery to prevent intracardiac thrombus in patients with atrial fibrillation. Herein, we report the surgical case of an 80-year-old man whose left atrial appendage became aneurysmic long after undergoing suture exclusion. At the age of 67, he underwent mitral valve annuloplasty and left atrial appendage suture exclusion for mitral regurgitation and chronic atrial fibrillation at our institution. At the age of 76, a mass near the left atrial appendage that was difficult to distinguish from a tumor was observed. During follow-up, the mass increased in size, and blood flow was observed inside. Consequently, surgery was performed when he was 80 years old. The interior of the mass was nearly filled with thrombus, and some oozing was observed;however, no blood flow was noted from the left atrium. The diagnosis was a left atrial appendage aneurysm that has developed over a long-term period of following suture exclusion. However, it was suspected to be a rare condition involving various factors, including residual left atrial feeding vessels, chronic expanding hematoma, and continuous use of anticoagulants.