A case of lymphangioma of the duodenum with a successful preoperative diagnosis is reported herein. A 76-year-old man who complained of tarry stool was found to have a hemorrhagic polypoid tumor in the third portion of the duodenum. The tumor was diagnosed histologically as cavernous lymphangioma by endoscopic biopsy. Since such a tumor is essentially benign, a partial resection of the duodenum including the tumor was performed. Therefore, an endoscopic biopsy seems to be valuable in the diagnosis of duodenal lymphangioma.