Transthoracic echocardiography (TTE) has substantial limitations for the study of abnormalities of the coronary tree. Transesophageal echocardiography (TEE) allows a more complete examination of the coronary arteries, particularly the proximal segments. This report describes the use of TEE after cardiac catheterization in the clinical management of a patient with unstable angina. While angiography first showed the giant aneurysm of the left circumflex coronary artery. TEE, by revealing an active thrombus of the lumen, prompted an immediate surgical resolution.