The emergency evaluation of a patient with acute life-threatening gastrointestinal hemorrhage requires the coordinated efforts of medical, surgical, and radiologic personnel. In most patients with an acute upper gastrointestinal hemorrhage, endoscopy represents the primary diagnostic procedure. Arteriography may follow, depending on the identification of the lesion at endoscopy or the need for therapy through the vascular catheter. Arteriography should precede endoscopy when bleeding is massive or the clinical situation suggests that therapy by a catheter must be instituted on a more urgent basis. When personnel are available to perform endoscopy and arteriography, the barium examination of the upper gastrointestinal tract is best postponed for a few days until the patient can be stabilized completely. In the patient with massive red rectal bleeding and a negative nasogastric aspirate, the arteriogram is clearly the procedure of choice for the emergency diagnostic evaluation. Bleeding diverticular and vascular ectatic lesions can be diagnosed only by arteriography and treatment can usually begin immediately by a catheter infusion of vasopressin. Colonoscopic and barium enema examinations of the acutely bleeding patient are not reliable. In addition, the presence of barium in the colon interferes with arteriography for a number of hours. The barium examination of the colon should be performed at some time during the hospitalization to exclude other possible bleeding lesions and to provide a better overall anatomic assessment of the colon. Both the retrograde and antegrade small bowel enemas are useful in detecting obscure small bowel bleeding lesions that occasionally present as a massive lower gastrointestinal hemorrhage.