Massive bleeding from a submucosal artery and small mucosal erosion has been termed Dieulafoy's ulcer or Exulceratio Simplex. The condition was originally described in the stomach but there have been reports of three similar lesions found in the colon. A patient with massive bleeding from a submucosal artery in the ascending colon with complete angiographic and histologic examination is the fourth case reported in the literature. This lesion is identical to previous clinical and pathological descriptions of Dieulafoy's ulcers presenting in the stomach. This pathologic entity may exist in the colon and can cause massive bleeding. Diagnosis is by angiography and by pathologic confirmation of the lesion. Treatment is primarily surgical resection because the massive bleeding precludes colonoscopic control of bleeding. The differential diagnosis of massive colonic hemorrhage should include this lesion.