Platypnea-orthodeoxia syndrome (POS) is a rare condition of positional hypoxemia and dyspnea. The following is a case of hypoxemia for no obvious reason in the perioperative scenario. A 70-year-old male patient was submitted to a radical prostatectomy. On several occasions after anesthetic induction, peripheral oxygen saturation decreased without any understandable cause. In the postanesthesia care unit, severe hypoxemia occurred in the upright position, oddly correcting with recumbency. An echocardiography showed a right-to-left intracardiac shunt through a patent foramen ovale with no pulmonary hypertension, establishing POS. Achieving this diagnosis required a broad workup with a high degree of suspicion.