Aneurysms have been recognized and treated since the time of Galen, in the second century. The articles that were written on this subject before 1800 were collected by Erichsen1 for the Sydenham Society. Klotz,2 in 1926, reviewed the general subject of aneurysm completely, and Fearnsides3 reviewed the history of intracranial aneurysm thoroughly. Those who are interested in the early history of this subject should consult these sources. MATERIAL The material for the present study included all the cases in which a diagnosis of aneurysm was seriously considered at the Mayo Clinic in the years 1925 to 1935, inclusive, except those in which there was an arteriovenous aneurysm or an aneurysm of the chambers of the heart. In this series of 596 cases the aneurysms have been divided roughly into five anatomic groups (fig. 1): intracranial aneurysms, intrathoracic aneurysms (including those of intrathoracic vessels other than the aorta),