Exracorporeal shock wave lithotripsy (ESWL) may require immersion of the patient in water and extradural blockade for analgesia, which may both affect pulmonary and cardiovascular function and increase the likelihood of hypoxaemia. Twenty-four patients (ASA l-III) with extradural blockade and six healthy subjects were studied by pulse oximetry during immersion. Oxygen saturation (Sa_O₂) tended to decrease after induction of extradural analgesia. Immersion decreased Sa_O₂ further, by up to 2% in healthy volunteers (P < 0.05) and 3% in patients (P < 0.05). In two patients with arteriosclerosis, an Sa_O₂ of 92% was accompanied by arrhythmias which resolved after administration of oxygen.