We investigated the factors implicated in the pathogenesis of anastomotic aneurysm formation and the postoperative course of patients with such a complication. Forty-five patients with 49 anastomotic aneurysms were diagnosed and treated in two vascular surgery departments in Athens, Greece, during an 8-year period. Emergent complications occurred in 15 cases, rupture in 11, and thromboembolic episodes in another four. Preoperative diagnostic workup in the remaining elective cases (n = 34) included color duplex scan, computed tomographic scan, and angiography. All patients underwent operation, and cultures were obtained during the surgical procedures. Histological examination of the host artery wall adjacent to the aneurysm was also performed. Aortobifemoral bypass was the original operation performed in the majority of cases (71%), and the femoral anastomosis was the most frequent site involved (85.7%). Emergent procedures were associated with increased mortality (46.6%), whereas elective operation resulted in high patency rates and no mortality. In an attempt to isolate predisposing factors that contributed to aneurysm formation, we concluded that the etiology was multifactorial in approximately one-third of the cases and degenerative host artery disease was the main cause (40%). Arterial degeneration is the leading cause of anastomotic aneurysm formation, and emergency arterial reconstruction in cases of aneurysm complication is associated with a poor prognosis.