To evaluate the usefulness of transesophageal echocardiography in thoracic aortic disease, transesophageal echocardiography was performed in 11 patients with thoracic aortic disease (true aortic aneurysm in two patients, aortic dissection in nine patients). Findings obtained by transesophageal echocardiography were compared with those obtained by computed tomography (nine patients) and angiography (six patients). Results were as follows: 1. True aortic aneurysm: Although transesophageal echocardiography and computed tomography could evaluate the size of the aorta and detect mural thrombus, angiography underestimated degree of aortic dilatation because of the presence of mural thrombus. 2. Aortic dissection: Although computed tomography could not differentiate thrombus in the false lumen from mural thrombus in two patients, transesophageal echocardiography could discriminate those two easily in one patient from properties and motion of the intimal flap, and flow information in the false lumen. Angiography could not evaluate the range of dissection in any patients with thrombus in the false lumen. With transesophageal echocardiography we could detect the entry in all six patients with aortic dissection of DeBakey type III except 1 patient with aortic dissection of DeBakey type I. Transesophageal echocardiography could obtain real-time image of most of the aorta except for a part of the ascending aorta. Therefore, transesophageal echocardiography was useful for diagnosing thoracic aortic disease and distinguishing between true aortic aneurysm and aortic dissection.