A case of villous adenoma of the duodenum, with focal in situ carcinomatous changes, has been described with a review of forty-two other case reports from the world literature. Occult bleeding, resulting in anemia, and vague obstructive symptoms appear to be the most common presenting findings. The average age was 56.4 years, which was seven years younger than the average age for villous tumors of the colon. Adequate radiologic studies should establish the diagnosis preoperatively. These tumors obtain relatively large size before causing significant symptoms. Approximately one third showed carcinomatous changes, and approximately one half of these were in situ changes. Local segmental resection for duodenal villous tumors is desirable when possible. However, in areas where this is not feasible, local mucosal excision is acceptable for benign tumors and for those with in situ carcinoma. If invasive carcinoma is found in the excised specimen, pancreatoduodenectomy is recommended. Insufficient evidence is available to adequately evaluate survival for malignant villous tumors of the duodenum, but the available data suggest that the survival after treatment of malignant villous tumors is comparable to other malignant lesions originating in the duodenum.