We describe an 82-yr-old patient with platypnea-orthodeoxia without identifiable lung disease in whom the diagnosis was suspected because of clinical history, echocardiography, and orthostatic measurement of arterial blood gases. Recumbent and upright cardiac catheterization techniques confirmed the presence of orthostatic variation in the degree of right to left shunt across the fossa ovalis. This was successfully treated by surgical closure of the interatrial communication in the region of the fossa ovalis.