Summary The introduction of the pulse oximeter has increased our awareness of post-operative hypoxaemia which is common and may be profound after surgery. Post-operative hypoxaemia may be divided into early and late phases. The effect of anaesthetic and analgesic drugs on both the maintenance of the upper airway and ventilation are important causes of early post-operative hypoxaemia. Upper airway obstruction as a result of laryngospasm, laryngeal oedema, vocal cord paralysis, arytenoid dislocation tracheomalacia and bleeding are less common causes. Late hypoxaemia is a result of a combination of impaired gas exchange and abnormalities of respiratory control induced by a combination of sleep and opiates. Control disturbances may result in both apnoea and upper airway obstruction. It is clear from long term oximetry studies that prolonged severe post-operative hypoxaemia occurs in many patients, particularly after upper abdominal surgery. It is less clear how significant this is in patients without cerebrovascular and cardiovascular disease. The use of long-term oxygen administration is discussed.