OBJECTIVE Brunner's gland adenoma is an extremely rare but important entity. Controversy exists over its etiology and pathogenesis, but the present view is that it is a duodenal hamartoma with a predominance of Brunner's gland elements. METHODS A case of a 76-year-old woman with a reddish pedunculated polyp prolapsing between the bulb and the second part of the duodenum seen at endoscopy and removed surgically through a longitudinal duodenotomy is presented. RESULTS The cut surface of the tumor had a grayish color, revealing multiple cystic spaces which on microscopic examination proved to be enlarged Brunner's glands. The hyperplastic glands formed lobules which were surrounded by bundles of fibromuscular and connective tissue. In the adjacent duodenum, large numbers of lobules of well-differentiated Brunner's glands with mucus-secreting epithelial cells were seen. CONCLUSIONS The reported case supports the theory that Brunner's gland adenomas are duodenal hamartomas with a predominance of Brunner's gland elements and further shows that a continuity exists in Brunner's glands of the tumor and those of the adjacent duodenum.