We compared a group of 52 patients with acute lower extremity ischemia secondary to arterial thrombosis with a series of 220 patients with peripheral embolism who were seen at the Massachusetts General Hospital, Boston, from 1967 through 1980. The conditions of patients with arterial thrombosis were misdiagnosed as acute embolism at a rate of 20%. Mortality in the patients with embolism was significantly higher. While patients with acute embolism were routinely treated with embolectomy, two thirds of the patients with thrombotic occlusion required surgery during their initial hospitalization for ongoing severe ischemia. More than half of these operations were carried out as emergency procedures. Limb salvage in surgically treated patients with arterial thrombosis was inferior to that seen with embolism. Early revascularization for acute thrombosis was carried out with a 30-day patency rate of 82%. While the mortality associated with acute thrombosis was significantly lower than that seen with peripheral embolism, the risk of major amputation was 35%. There should be no reluctance to proceed with indicated vascular reconstruction in the setting of acute limb ischemia secondary to arterial thrombosis.