A collection of barium simulating an ulcer, or a negative defect simulating a mass, frequently occurs at the junction of the first and second portions of the duodenum. This appearance may be related to several factors, including (a) distortion produced by flexion, (b) sphincter function, and (c) an extrinsic impression. Several such patients have had negative findings on endoscopy. The defect can be distinguished from a true lesion by its characteristic location and its variable appearance on different projections.