An 82-year-old woman presented with unexplained hypoxia that had persisted for several years; dyspnea worsened while in an upright posture. We diagnosed platypnea-orthodeoxia syndrome (POS) because her symptom improved while in the lying down position. Cardiac angiography confirmed a right-to-left shunting across a patent foramen ovale (PFO) without an elevated right-left pressure gradient. We speculated that an increase in right-to-left shunting through PFO with blood flow from the inferior vena cava may account for POS under kyphosis due to aging. After a percutaneous transcatheter closure of the PFO performed with the Amplatzer septal occluder, her peripheral oxygen blood saturation levels improved in the upright position. In conclusion, for elderly patients with unexplained hypoxia, we should consider altering the peripheral oxygen blood saturation by changing the patient's position.