A 41-year-old man with mucosal prolapse syndrome received endoscopic ultrasonography. The lesion was mildly hypoechoic and irregular, with extension into the muscularis propria and disappearance of the normal mucosal and submucosal layers. In addition, an enlargement of the muscularis propria, bridging towards the mucosal layer with merging of the two layers, was observed. These findings suggest the usefulness of endoscopic ultrasonography in the diagnosis of mucosal prolapse syndrome.