A 75-year old man was referred to hospital for symptomatic hypoxemia. He did not complain of dyspnea while supine, but while sitting or standing, he experienced dyspnea with severe hypoxemia. He did not have any pulmonary diseases that could cause dyspnea. Transesophageal echocardiography revealed an atrial septal aneurysm with a small atrial septal defect (ASD) and a mild left-to-right shunt through the ASD when the patient was supine. However, when he became upright, a severe right-to-left shunt occurred and the arterial oxygen saturation decreased from 96% to 80% with dyspnea. Cardiac catheterization revealed normal pulmonary artery pressure. He was therefore diagnosed as having platypnea - orthodeoxia syndrome. Magnetic resonance imaging of the chest showed a deformity of the atrium associated with elongation of the ascending aorta. The ASD was closed surgically and the dyspnea and hypoxemia that occurred while he was upright completely resolved.