A 77-year-old woman developed dyspnea over three years which occurred during sitting, standing or walking. Her physical examination, chest X-ray, ECG and cardiac catheterization results were all normal. A marked fall in arterial oxygen saturation was observed on sitting or standing. Transesophageal echocardiography showed an increase of right to left shunt flow on sitting. The patient was diagnosed with platypnea-orthodeoxia syndrome and underwent the surgical closure of an atrial septal defect of 19 mm in diameter. After the surgery, the patient's POS symptoms were completely resolved. She was discharged and followed at the outpatient clinic. Her post-treatment course was uneventful.