Two cases of orthostatic dyspnea and arterial desoxygenation, as a postpneumonectomy complication, are reported. In one patient, echocardiography and cardiac catheterization revealed the presence of an atrial septal aneurysm, which has never been described in association with the platypnea-orthodeoxia syndrome. The other case illustrates that, despite actual technical possibilities, clinical suspicion remains a prerequisite for proper diagnosis of the entity.