Idiopathic myointimal hyperplasia of the mesenteric veins (IMHMV) is a rare cause of intestinal ischemia secondary to venous compromise. A patient with this condition who presented with crampy abdominal pain, diarrhea, and rectal bleeding initially attributed to inflammatory bowel disease had several colonoscopies and ultimately a sigmoid colectomy. The colonic mucosa in biopsies performed at initial presentation and subsequently and in the resection specimen contained numerous hyperplastic, thick-walled, hyalinized vessels in the lamina propria, which have not been described in this entity previously. Examination of the mucosa in 27 resection specimens of ischemic enterocolitis of various etiologies, in five resections of prolapsed rectum, and in seven colostomy specimens revealed no instance in which there were similar histologic abnormalities. When seen on biopsy, therefore, these features should lead to inclusion of IMHMV in the differential diagnosis. Furthermore, the characteristic lesions of the submucosal and extramural veins in IMHMV were compared with those of 14 examples, from several organs, of veins subjected to arterial pressure and 21 cases of venous hypertension. The marked similarity of the arterialized veins to the mural veins of IMHMV suggests a role for arteriovenous fistulization in the pathogenesis of IMHMV, and a mechanism by which this might occur is proposed.