INCREASING use of arterial homografts in the surgical treatment of various forms of arterial disease has emphasized the need for additional information concerning the biologic properties of transplanted vessels. The functional integrity of arterial homografts as well as the structural changes they undergo after transplantation in experimental animals has long been well known.* More recent experimental studies have amplified these observations, particularly as regards the influence of various methods of graft preservation.† Similar studies in man, however, have not been available. It seems desirable, therefore, to report our studies on the structural changes occurring in human aortic homografts from one day to one year following transplantation. These observations are based upon 10 cases in which aortic bifurcation homografts were used to replace excised segments of the abdominal aorta, for arteriosclerotic aneurysms in 8 instances and for thrombo-obliterative disease in 2. The technique used in the performance of this operative procedure