The remarkable development of surgery of the chest has greatly increased interest in mediastinal tumors. The removal of such tumors is now a common event. In general it has been my own custom as well as that of others to explore, with the idea of removing them, those which have not responded to roentgen irradiation, unless they are considered to be aneurysms. An unexpected experience in operating on 2 patients with aneurysm of the ductus arteriosus who did not give positive evidence of aneurysm has made me think that the experience should be recorded. This record seems especially desirable because, on the basis of these 2 cases, it would seem as if many of the features are sufficiently characteristic to make one at least suspicious of the presence of an aneurysm of the ductus arteriosus even when no pulsation can be made out. These features will be discussed later.