A 73-year-old woman with interstitial pneumonia was admitted with dyspnea and hypoxemia. Chest CT and a pulmonary function test showed no progression of the interstitial pneumonia, and heart failure and pulmonary embolism were excluded. After admission, the patient had dyspnea and hypoxemia in a sitting position, but not in exertion or in a supine position. A transesophageal echocardiography revealed blood flow through the foramen ovale in a sitting position. We diagnosed platypnea-orthodeoxia syndrome (POS) because of an interatrial right-to-left shunt. POS is defined as dyspnea and hypoxemia induced by the sitting position. In this case, she had an elongated ascending aorta and kyphosis, which may cause abnormal orientation of the patent foramen ovale. POS is a rare disease, and careful examination at what position and when dyspnea and hypoxemia appear is an important clue in the diagnosis of hypoxemia.