The cause of bleeding was found in 8 of 10 cases of colonic diverticulosis associated with severe rectal hemorrhage by using arteriographic and microangiographic techniques to localize the site and serial histological sections to study the lesion. Strikingly consistent changes were identified. These are related to the characteristic angioarchitecture of colonic diverticula. These changes include asymmetric rupture of the vas rectum toward the lumen of the diverticulum precisely at its dome or anti mesenteric margin; conspicuous eccentric intimal thickening of the vas rectum, often with medial thinning and duplication of the internal elastic lamina at and near the bleeding point; and general absence of diverticulitis. Control colonic diverticula demonstrated normal structures or, only occasionally, minimal eccentric intimal thickening in their associated vasa recta. This suggests that traumatic factors arising within the diverticular or colonic lumen induce asymmetric intimal proliferation and scarring of the associated vasa recta, predisposing to rupture and massive bleeding.