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BACKGROUND Idiopathic myointimal hyperplasia of the mesenteric veins is a segmental ischemia associated with non-inflammatory hyperplasia of the intimal smooth muscle of the mesenteric veins. Owing to its rarity, timely diagnosis is often difficult. OBJECTIVE To improve clinical practice in terms of the diagnosis of idiopathic myointimal hyperplasia of mesenteric veins. DESIGN A retrospective observational study. SETTINGS Single institution, case collection from clinical archives. PATIENTS The data from 12 idiopathic myointimal hyperplasia of the mesenteric veins cases were retrieved during 2006-2020. Most patients were elderly men, with a male-to-female ratio of 10:1. MAIN OUTCOME MEASURES Clinical, endoscopic, radiologic, and pathologic characteristics of idiopathic myointimal hyperplasia of mesenteric veins. RESULTS Radiologically, marked segmental mural thickening and poor enhancement involved the sigmoid colon and rectum in most cases, with extension to the descending colon in some cases. Typical cases showed obliteration of the inferior mesenteric veins and collateral vessels. Colonoscopic findings were reminiscent of ischemia or ulcerative colitis, but sharp demarcation from the uninvolved segment was the most distinguishing feature. Surgically resected specimens showed marked segmental mural thickening, edema, and mucosal discoloration grossly. Microscopically, thick-walled, tortuous veins were observed mainly in the submucosa and subserosa and submucosa was markedly thickened in all cases. The subserosal large veins showed myointimal hyperplasia and pericolic fat necrosis was invariably observed. The most useful histologic finding in biopsy material was tortuous, arteriolized mucosal capillaries with occasional fibrinoid necrosis. LIMITATIONS Small number of cases; Selection bias; No prospective external validation. CONCLUSIONS Radiologic and pathologic features of idiopathic myointimal hyperplasia of the mesenteric veins are distinct from those of ulcerative colitis or non-specific ischemic colitis. Careful interpretation of endoscopic and radiologic images and generous biopsies with interpretation by experienced pathologists might lead to an early diagnosis and prevent unnecessary medical treatment. See Video Abstract at http://links.lww.com/DCR/B806 .