A 37-year-old woman was transferred to our department for surgery. She had experienced intermittent chest discomfort or pain and palpitation during the last 4 years. Chest radiographs showed cardiomegaly (cardiothoracic ratio 0.63). A preoperative ECG showed normal sinus rhythm (Figure 1). Echocardiography showed marked dilatation of the coronary sinus with an intramural thrombus and flow acceleration at its opening (Figure 2). Cardiac catheterization showed no significant coronary artery stenosis but revealed coronary arteriovenous fistulas (CAVFs) from the distal right coronary artery, distal left anterior descending artery, and proximal left circumflex artery draining directly into the coronary sinus (Figure 3). A preoperative computed tomography scan showed a large aneurysm that contained thrombi with a maximal diameter of 73 mm at the outer side of the left ventricular inferior wall (Figure 4). The site of the aneurysm coincided with the coronary sinus and greater cardiac vein. The computed tomography scan was unable to reveal drainage of the thrombosed aneurysm into the right atrium (RA). Magnetic resonance imaging, however, showed a pinpoint opening from the coronary sinus aneurysm to the RA (Figure 5). The right coronary artery was diffusely dilated and was connected to the coronary sinus after forming a small coronary artery aneurysm. Distal collateral vessels from the left anterior descending artery and left circumflex artery were connected directly to the coronary sinus (Figure 6). On …