Dyspnoea after pneumonectomy is considered to be rarely the result of an intracardiac shunt. We report on four patients who were suffering from the rare syndrome of breathlessness and hypoxaemia in the upright position after right pneumonectomy because of an open foramen ovale with a right to left shunt, although pressures in the right heart were normal. All patients showed striking arterial desaturation which responded insufficiently to breathing 100% oxygen. The arterial saturation became normal after surgical closure of the defect.