Pathomorphological aspects of arteriosclerosis obliterans were reviewed. While the morphology is basically atherosclerosis, complete occlusions by fresh or old thrombi are often observed. Obstruction with lipid-poor fibrous tissue is also recognized. Calcification of intima or media is often present, but it does not correspond with grade of atherosclerosis. Occlusion of major arteries of the lower limbs are frequently discovered in asymptomatic aged persons. The occlusive lesions usually far-advanced before initial clinical manifestation of ischemic processes becomes apparent. Before clinical manifestations, various compensatory mechanisms including arterial anastomosis and decrease of the peripheral vascular resistance react to correct the abnormal arterial circulation. Clinical initiation first appears after the failure of these compensatory mechanisms. As in other atherosclerotic diseases, etiology of arteriosclerosis obliterans must be diverse, but upright gait of man may have some influence to develop the disease. Among many aggravating factors, diabetes mellitus is the most serious and it shows several features different from ordinary arteriosclerosis oblilerans.