We report the case of a 71-year-old man bearing a severe right-to-left shunt through a patent foramen ovale in the absence of elevated right-sided heart or pulmonary artery pressures. He presented with platypnea-orthodeoxia syndrome, but he had no pulmonary or extracardiac diseases that are known to be associated with this syndrome. Chest radiography showed a bulky aneurysm of the thoracic aorta. A peripheral contrast transesophageal echocardiography demonstrated a large right-to-left shunt through a patent foramen ovale. In addition, the atrial septum was severely deformed by an aneurysm including this patent foramen ovale. We hypothesized that the opening of the foramen ovale was the result of a mechanical deformation of the atrial septum by two contributing factors: the aneurysm of the thoracic aorta and the aneurysm of the septum itself.