Transesophageal echocardiography (TEE) provides a rapid and accurate diagnosis in patients with dissection of the thoracic aorta. The procedure is considered to be safe. We report a case where rupture of a dissecting thoracic aneurysm occurred during attempted TEE in a patient, who had been clinically stable for 1 week. TEE was performed because aortic valve endocarditis was suspected, and the procedure did not include close surveillance of the blood pressure. Careful manipulation of the probe, adequate sedation, monitoring, and treatment of acute rises in blood pressure was recommended in patients with even a slight suspicion of dissection undergoing TEE. (ECHOCARDIOGRAPHY, Volume 13, July 1996)