Non-atherosclerotic coronary artery aneurysms are rare and most of them remain asymptomatic. We report a case who has a giant circumflex coronary artery aneurysm with fistulisation into the coronary sinus. The patient presents with dyspnea and palpitation due to atrial fibrillation with rapid ventricular response. The diagnostic contributions of echocardiography, coronary angiography are discussed. The hemodynamic effects of this anomaly are reviewed.