BackgroundHiatal hernias are common. In some reports, hiatal hernias have been implicated in causing dyspnea, syncope, and heart failure.Case presentationAn 82-year-old woman with a hiatal hernia was admitted to our hospital because she had experienced postprandial syncope during the last few years. Esophagogastroduodenoscopy revealed a large hiatal hernia and a pedunculated polyp of the stomach antrum that fit into the pylorus. An upper gastrointestinal contrast study showed that the entire stomach had relocated to the thoracic cavity and that the body of the stomach was located above the fundus, resulting in a so-called upside-down stomach. Contrast-enhanced computed tomography revealed that a large portion of the stomach, transverse colon, and part of the pancreas were present in the mediastinum. We then performed transthoracic echocardiography followed by a water pouring test using a nasogastric tube. After instillation of 2000 ml of saline, the left atrium was markedly compressed and the area of the mitral annulus was reduced. We determined that stomach dilation by the hiatal hernia and gastric polyp had caused the syncope. The patient underwent laparoscopic hiatal hernia repair and endoscopic gastric polypectomy, and she experienced no syncopal episodes for 5 months postoperatively.ConclusionsClinicians should recognize that a large hiatal hernia may be a risk factor for syncope.