Duodenal inflammatory fibroid polyps (IFP) are extemely rare lesions indistinguishable from submucosal tumors by endoscopic inspection alone. Like gastric inflammatory fibroid polyps, they can be managed by endoscopic polypectomy or mucosectomy. However, preoperative diagnosis of this benign lesion is difficult. Here we present a case of duodenal IFP causing gastrointestinal bleeding that was evaluated by endoscopic ultrasound before surgical removal. On endosonography, the duodenal IFP appeared as a coarsely heterogeneous isoechoic and hypoechoic mass circumscribed by a distinct margin and arising from the third layer of the duodenal wall. The endosonographic appearance of this lesion was in marked contrast to that previously reported for gastric IFPs, which have tended to appear as hypoechoic homogeneous lesions with indistinct margins. Endosonographic evaluation of suspected IFPs before endoscopic or surgical treatment is useful. However, the endosonographic appearances of duodenal and gastric IFPs may be significantly different, possibly because of differences in the makeup of the duodenal and gastric walls.