Introduction: Massive lower GI bleeding can be life threatening, especially when the source of bleed is not controlled. In this case, the large bowel was found to be necrotic due to intracolonic tamponading by blood from an ileac artery--colon fistula. Case Presentation: A 60-year-old woman was brought to ED after a pre-syncopal episode following a bloody bowel movement with cramping abdominal pain. She had history of Hodgkin's and non-Hodgkin's lymphoma s/p chemotherapy, stem cell transplantation and external beam radiation.The excised left external iliac lymph nodes had shown spread of malignancy. She was on warfarin for recurrent DVTs. She was afebrile, with BP 80/62 mmHg and PR 106/min. Her abdomen was distended with diffuse tenderness, and bowel sounds were present. Labs showed Hct: 25% (baseline 32%), Hgb: 8.5 g/dl, WBC: 14,000/mm3, platelets: 560,000/mm3, INR: 3.4, BUN: 23, creatinine: 1.0 and ALP: 350. The last colonoscopy was positive for hemorrhoids and adenomatous polyps that were resected. In the ICU, anticoagulation was reversed. History of malignant spread to the region prompted consideration of radiotherapy as the cause of colitis or bowel necrosis. The patient did not tolerate the colonoscopy preparation. Tagged RBC scan demonstrated bleeding in the left colon. CT revealed free fluid with a massively dilated colon. Angiography, however, showed no extravasations. Overnight, she developed signs of peritonitis, mandating an emergent laprotomy. Ascending colectomy was performed for a non-perforated necrotic right colon distended with fresh blood and clots. Post operatively, she became pressor dependent, the wound VAC drained frank blood, hematocrit dropped further, and ten units of PRBCs were transfused in total. Repeat laprotomy showed blood in the abdomen with massively dilated and necrotic left colon. Pressure on the left common iliac artery controlled the bleeding. Intra-operatively, a fistula was found between the left external iliac artery and the friable left colon; this was cauterized. A femoro-femoral graft was constructed and the rectal stump was connected to a drain. Discussion: The prevalence of iliac artery-enteric fistulas is not well documented. They are described mostly secondary to malignancies and radiotherapy. In this case, radiotherapy for metastatic lymph node disease led to bowel necrosis and fistula formation. Selective mesenteric angiography is insufficient to demonstrate bleeding from the iliac arteries, but in our case, a negative preoperative angiogram was also due to a blood tamponaded colon. Suspected early, bowel necrosis could have been prevented. Definitive management entails bowel resection, arterial ligation and extra-anatomic bypass.