Acute aortic thrombosis is an unusual entity related to atherosclerotic disease by etiology, but mimicking saddle embolus as to course. Our nine patients with acute aortic thrombosis illustrate a characteristic sequence of established claudication followed by sudden, unrelenting rest pain, severe ischemia, absence of pulses below the umbilicus, and (usually) no apparent cardiac source for an embolus. Eight patients underwent aortoiliac or aortofemoral bypass. Two patients (25%) died, one of preexisting renal failure and the other following a graft infection. Disregarding operative mortality, bypass results in a 94% limb salvage rate. The ninth patient presented in extremis, and demonstrated the natural course of the disease with hyperkalemia as the ultimate cause of death. Nonoperative therapy results in death. Conservative surgical efforts are equally futile. Urgent aortoiliac or aortofemoral bypass is considered mandatory.