A 78-year-old man with a history of gastric ulcer and pulmonary embolism was admitted for elective revision of a right total hip replacement. He was mildly hypoxic preoperatively (saturation 89% on air). He became profoundly breathless postoperatively (saturation 75%). He was treated for presumed pulmonary oedema but failed to improve. A CT pulmonary angiogram and transthoracic echo showed no clear cause for his symptoms. Because the patient's symptoms were postural, exacerbated in the upright position and relieved by lying supine, the authors suspected a diagnosis of platypnoea-orthodeoxia syndrome associated with a patent foramen ovale (PFO). Transoesophageal echo and microbubble study confirmed he had a PFO. The patient's PFO was percutaneously closed and his symptoms and positional hypoxia completely resolved.