A case of pseudocoarctation of the thoracic aorta manifested by a mediastinal mass on a roentgenogram of the chest and subsequently evaluated with computed tomography (CT) is presented. The following combination of CT findings is thought to be diagnostic of this rare congenital anomaly: (a) demonstration that the mass is part of the aorta, (b) the depiction of the unusual aortic arch high in the mediastinum, (c) visualization of the isthmus portion of the descending aorta not adjacent to the spine but rather located ventral to it and surrounding by aerated lung, and (d) more caudal origin of the subclavian artery resulting in a vascular shadow posterior to the kinked aortic arch.