Duodenal ulcer: A crater (ulcer) in the lining of the beginning of the small intestine (duodenum). Ulcer formation is caused by infection with Helicobacter pylori. Other factors predisposing a person to ulcers include anti-inflammatory medications and cigarette smoking. Ulcer pain may not correlate with the presence or severity of ulceration. Diagnosis is made with barium X-ray or endoscopy. Complications of ulcers include bleeding, perforation, and blockage. Treatment involves using antibiotics to eradicate H. pylori, eliminating risk factors, and preventing complications.