A 16 year-old Korean girl with cardiomegaly was found to have a "right atrial cystic mass" on transthoracic echocardiography. An unusual cystic structure made the diagnosis difficult. However, transesophageal echocardiography and multidetector computed tomography revealed a coronary artery fistula with a distal saccular aneurysm involving the interatrial septum and draining into the right atrium. Multidetector computed tomography provided clear anatomic visualization that fully delineated the abnormal structures. Multidetector computed tomography may be considered as a good alternative for transesophageal echocardiography in assessing a tortuous coronary artery fistula and aneurysm with complex anatomy.