The diagnosis of coronary aneurysms has already been described using transthoracic and transesophageal echocardiography. In the present report we demonstrated the use of real-time three-dimensional echocardiography for the diagnosis of a large left circumflex coronary aneurysm. The improved spatial resolution of this new imaging technique provided valuable information for anatomic characterization of the coronary aneurysm, allowing for distinction between the lumen and lining thrombus. The findings were confirmed by magnetic resonance imaging and coronary angiography.