A 44-year-old woman visited our clinic because of sudden onset pain in both lower extremities. After admission, computed tomography (CT) was performed and revealed occlusion of both popliteal arteries and renal infarction in the left kidney. Transthoracic echocardiography (TTE) revealed rheumatic mitral stenosis (Figure 1A). In the apical view of TTE and on transesophageal echocardiography, we identified a huge dilated coronary sinus (CS) (Figure 1B) and detected large defect between the dilated CS and left atrium (LA) (Figure 1C, Movie 1). On contrast echocardiography with agitated saline, the right atrium (RA) was filled with air bubbles, but not the dilated CS, and then the right ventricle was filled with air bubbles later (Figure 1D). This finding showed no persistent left superior vena cava (PLSVC), which was supported by the CT imaging finding (Figure 2A). On CT, a huge dilated CS was observed, showing no communication with the RA, which suggested CS orifice atresia (Figure 2B). On real-time three-dimensional (3D) echocardiography in the en face view of the LA, an ovoid-shaped defect was found between the CS and the LA, posterior to the mitral valve (Figure 3A). After horizontal rotation of the image, no communication between the CS and the RA was visible, which suggested CS orifice atresia (Figure 3B). The patient was discharged after stabilization.