Prolapse of the gastric mucosa into the duodenum must be considered when a round soft tissue mass is seen in the right upper quadrant on scout abdominal film. Gastric prolapse may mimic tumor in the duodenum when the prolapse is large. Examination with barium meal is necessary to exclude prolapse of the gastric mucosa into the duodenum as a cause of epigastric pain and vomiting. Medical treatment is suggested for patients with mild symptoms, but patients with severe symptoms, repeated hemorrhage, anemia, severe intermittent epigastric pain and vomiting due to ball-valve syndrome should have operation.