A case of primary mesenteric vein thrombosis with extensive small bowel infarct beginning at the ligament of Treitz presented as upper gastrointestinal bleeding. Although endoscopy of the upper GI tract disclosed erosive gastritis this finding was considered insufficient to explain the hematemesis; therefore the endoscope was advanced further until a necrotic and aperistaltic-looking mucosa was found in the area of the ligament of Treitz. This paper describes the endoscopic picture of necrotic duodenal mucosa, and stresses the importance of a deeper examination into the duodenum when a clear-cut cause for the bleeding is not found in the stomach or duodenal bulb.