Dissecting aneurysm of a right-sided descending aorta with a left-sided aortic arch.

A 66-year-old man presented with myocardial infarction. Chest X-ray showed a large mediastinal mass. Aortic dissection was suggested by a past history of chest trauma, but the mass was in a very atypical site. Dissection of an abnormally placed right-sided descending aorta was confirmed by computed tomography. The aortic arch lay on the left side. This rare combination of congenital and acquired heart disease led to diagnostic difficulty.