We report the case of a 74-year-old woman with a history of hypertension, hypercholesterolemia, and pacemaker who presented to the hospital with new onset New York Heart Association class IV congestive heart failure. Transthoracic echocardiography revealed a markedly dilated right ventricle with normal right ventricular systolic function. There was moderate pulmonary hypertension with an estimated pulmonary artery systolic pressure of 60 mm Hg. Her echocardiogram 1 year earlier had demonstrated normal right ventricular size and systolic function, and no pulmonary hypertension. Additional transthoracic imaging with saline contrast study through a left peripheral vein demonstrated the presence of a dilated coronary sinus with a persistent left superior vena cava. Color Doppler demonstrated turbulent flow within the coronary sinus with evidence of significant left-to-right shunting. Cardiac catheterization revealed a massively dilated left main coronary artery aneurysm with an arteriovenous fistula into the left superior vena cava and coronary sinus. The calculated Qp/Qs was 2:1. The patient underwent 2 unsuccessful attempts at percutaneous intervention to occlude the arteriovenous fistula. She then underwent successful surgical closure of the coronary arteriovenous fistula. The important role of intraoperative transesophageal echocardiography in guiding this technically challenging surgical case is discussed.