A 52-year-old man with a symptomatic condition-a huge left coronary artery fistula simulating a cor triatriatum sinister on transthoracic echocardiography-was referred to our center for surgery. Coronary angiography unequivocally established the diagnosis of a left coronary artery fistula and contrast opacification of the right atrium was observed. However, as a result of the size of the fistula, the entry site could not be defined with certainty. A transesophageal echocardiography (TEE) was then performed, and the opening site was clearly identified in the postero-lateral wall of the right atrium. After the surgical treatment transesophageal echocardiography was repeated, and it was confirmed that the surgical treatment had succeeded.