A 66-year-old male presented with exertional dyspnea and palpitations. The electrocardiogram showed atrial fibrillation and episodes of atrial flutter. Subsequent echocardiography revealed signs of right-sided heart failure with dilatation of the right ventricle and atrium, while left ventricular function was normal. In order to rule out coronary artery disease, coronary angiography was performed. The left coronary artery had atherosclerotic plaque without significant stenotic lesions, while the right coronary artery was chronically occluded with retrograde filling by collaterals. Attempting to catheterize the right coronary artery, the catheter entered a massively dilated vessel. Aortography confirmed a giant fistula of 13.4 mm average diameter connecting the aorta and the coronary sinus (Figure 1). A systemic blood flow (QS) of 5.6 l/minute and a pulmonary blood flow (QP) of 10.3 l/minute were calculated by hemodynamic and oximetric measurements, leading to a calculated left-to-right shunt of 4.7 l/minute (QP/QS = 1.8). Pulmonary artery pressure of 42/24 mmHg, combined with normal pulmonary capillary wedge pressure, verified pulmonary hypertension, most likely due to volume overload. A second-look echocardiography showed dilatation of the right coronary artery ostium and the coronary sinus ostium up to 26 mm. Cardiovascular magnetic resonance imaging, including three-dimensional reconstruction, confirmed the findings (Figure 2). Surgical treatment was recommended to the patient.