We report the case of a 61 yr old male, who developed a severe right-to-left shunt through a patent foramen ovale, in the absence of elevated right-sided heart pressures, two months after a left-sided pneumonectomy. This is considered to be a rare complication after pneumonectomy. However, taking into account the approximately 20% incidence of patent foramen ovale in the general population, we suggest that right-to-left shunting through an unsuspected foramen ovale or atrial septum defect should always be considered as a possible cause of otherwise unexplained hypoxaemia.