Brunner's gland adenoma is a rare tumor of duodenum. Patients are usually aymptomatic and most are discovered incidentally during the upper gastrointestinal (GI) series or esophagogastroduodenoscopy. These lesions are most commonly located in the duodenal bulb and clinical manifestations are variable. In symptomatic patients, the most common manifestations are GI hemorrhage and duodenal obstruction. On histologic examination, Brunner's gland adenoma that causes clinical symptoms is composed of hyperplastic Brunner's glands and contains mostly an admixture of glandular, adipose, and muscular tissues. We report a case of large Brunner's gland adenoma causing upper gastrointestinal hemorrhage in a 47-year-old woman which was successfully removed by endoscopic resection without complications such as bleeding or perforation. Microscopically, it was entirely composed of variable Brunner's glands.