The duodenal bulb (also ampulla of duodenum, duodenal ampulla, or duodenal cap) is the initial, dilated portion of (the superior part of) the duodenum^[1] just distal to the stomach; it begins at the pylorus and ends at the neck of the gallbladder. It is normally about 5 centimeters long.^[2]

The duodenal bulb is the site of duodenal ulcer occurrence. Duodenal ulcers are more common than gastric ulcers and - unlike gastric ulcers - are caused by increased gastric acid secretion. Duodenal ulcers are commonly located anteriorly, and rarely posteriorly. Anterior ulcers can be complicated by perforation, while the posterior ones bleed. The reason for that is explained by their location. The peritoneal or abdominal cavity is located anterior to the duodenum. Therefore, if the ulcer grows deep enough, it will perforate, whereas if a posterior ulcer grows deep enough, it will penetrate the gastroduodenal artery and bleed.^{[citation needed]}