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Idiopathic myointimal hyperplasia of mesenteric veins causes chronic ischemic mucosal injury with segmental strictures that mimic inflammatory bowel disease and nonocclusive ischemic colitis. It is characterized by myointimal proliferative changes that narrow the lumina of veins combined with ischemic injury and ulcers. Most cases reported to date have been diagnosed following surgical resection. The aim of this study was to determine whether mucosal changes of idiopathic myointimal hyperplasia of mesenteric veins are sufficiently sensitive and specific to allow its recognition in biopsy material. The study group consisted of 10 patients with idiopathic myointimal hyperplasia of mesenteric veins who underwent surgical resection of the affected colon, 7 of whom had available prior endoscopic biopsies. The control group included 10 patients each with radiation, nonocclusive ischemia, Crohn disease, diverticulitis, and mucosal amyloidosis, and 5 cases of small vessel (leukocytoclastic) vasculitis. Study patients were mostly older men with distal colorectal disease. All resection specimens showed mucosal ischemia with numerous thick-walled (arteriolized) capillaries and glassy subendothelial fibrin deposits; numerous hyalinized, eosinophilic thrombi were detected in 90% of colectomy specimens. Biopsies showed arteriolized capillaries (100%), subendothelial fibrin deposits (86%), fibrin thrombi (43%), and perivascular hyalinization (43%). Fibrin thrombi were observed in only one case each of ischemic colitis and small vessel vasculitis, and none of the other abovementioned features were seen in any of the controls. We conclude that arteriolized capillaries, subendothelial fibrin deposits, and perivascular hyalinization are frequent and specific features that can facilitate recognition of idiopathic myointimal hyperplasia of mesenteric veins in biopsy samples.