A 70-year-old woman was referred for an evaluation of evidence indicating occult bleeding found in the feces and epigastralgia. Radiologic and endoscopic examinations disclosed a duodenal polyp at the bulb. A pathological examination of a biopsied specimen revealed a well-differentiated adenocarcinoma. The patient underwent a gastrectomy and an extensive duodenectomy, and the surgical specimen examined indicated a pedunculated polyp (1.0*2.0) at the duodenal bulb which, histologically, consisted of two distinct parts: one, a tubular adenocarcinoma, and the other, non-tumorous tissue. Most of the lesion was the former, whereas the latter was found at the bottom and at the stalk of the polyp, and consisted of hyperplastic foveolar structures with gastric type cells and pyloric type glands. The histological findings of the mucosa, which was clearly duodenal around the root of the polyp, indicated that the carcinomatous portion was confined to the mucosal layer. These findings indicate that the adenocarcinoma in this case originated from a duodenal polyp composed of aberrant gastric mucosa. Also included in this study is a review of 57 cases of early duodenal cancer of the bulb, that have been reported in Japan.