We treated a 77-year-old woman diagnosed with severe aortic stenosis, who had undergone catheter ablation for paroxysmal supraventricular tachycardia at the age of 62. Although the patient remained in sinus rhythm, she had been receiving Coumadin with a target INR level of 2.0 since that time. Preoperative coronary angiography revealed a coronary-left atrium (LA) fistula, while computed tomography and echocardiography findings did not detect thrombus formation. Intra-operative transesophageal echocardiography revealed a 2-cm solid mass, which unexpectedly appeared during deairing manipulation. Prompt cross-clamping and removal of an LA thrombus with closure of the LA appendage contributed to an uneventful postoperative course. The LA should be explored if a coronary-LA fistula is noted, even in non-mitral cases, especially those with a history of catheter ablation.