This report is concerned with an analysis of 16 anastomotic aneurysms which developed in 12 patients after arterial surgery in which dacron sutures were used for all graft anastomoses. Knitted dacron grafts had been used in all but two of the original surgical procedures; a woven dacron graft had been used in one procedure and a saphenous vein graft in the other. The majority of anastomotic aneurysms occurred in the femoral triangle at the site of an end-to-side anastomosis. Five anastomotic aneurysms were caused by infection, and a variety of etiologic factors were responsible for the 11 non-infected aneurysms. Fourteen anastomotic aneurysms in 10 patients were treated surgically. A good result had been obtained in six patients; one patient required amputation of an extremity and three patients died postoperatively as a result of cardiac disease or pulmonary embolization. This report also contains a discussion of the factors affecting the occurrence of anastomotic aneurysms and a description of specific operative measures designed to decrease the incidence of these aneurysms.