Occlusion of the minute vessels of the bowel produces hemorrhage, ulceration, and necrosis of the bowel wall. To clarify the pathophysiology of the ischemic lesions caused by minute vessel occlusion, a microbarium suspension was injected into the mesenteric vessels of mongrel dogs. The pathological process in the bowel wall was parallel to the degree and extent of occlusion. The changes observed in these experimental studies were similar to those encountered in patients with ischemic enterocolitis. From our clinical and experimental analyses, the bowel lesions caused by minute vessel occlusion were classified into 3 categories according to the layer of the bowel wall involved, namely (a) mucosal, (b) submucosal and muscular, and (c) all layers of the bowel wall. In the lesions confined to the mucosal layer, the pathological processes were transient and reversible, and special treatment was not necessary since spontaneous recovery could be expected in almost all cases without any functional disturbance. When the ischemic changes involved the submucosal layer beyond the base of the crypts, ulceration almost always developed and was followed by a high incidence of stenosis due to scar formation. In this type of lesion, surgical treatment is required if stenosis persists after healing of the intestinal isehemic lesions. When the ischemic processes extended through all layers of the bowel wall, characteristic endoscopic findings were observed even in the early stages. Because of the high mortality rate, immediate surgical intervention is mandatory.