Continuous pulse oximetry monitoring was used to determine the incidence of hypoxaemia (arterial oxygen saturation less than or equal to 90%) occurring in the first hour of postoperative recovery. Of 107 patients studied, hypoxaemia was recorded in 80%. Twenty-eight (26%) of these patients had saturations below 80%. The average frequency (i.e., the number of desaturations per patient) and the total duration of these desaturations was 7.7 desaturations and 182 seconds respectively. Intermittent measurements taken preoperatively and at 5 and 30 minutes postoperatively revealed hypoxaemia in 2%, 4% and 6% of patients respectively. In 39 patients who received oxygen therapy throughout the monitoring period, 64% experienced hypoxaemia within the first 15 minutes of recovery as opposed to only 18% in the final 15 minutes monitoring period. Of the factors assessed, only patients with a body mass index greater than 25 had an increased risk of hypoxaemia (P less than 0.01). Four patients required active intervention and ventilatory assistance. We conclude that postoperative hypoxaemia is a particularly common occurrence even in patients otherwise considered healthy. Hence, pulse oximetry should be employed routinely during recovery. Where possible, monitoring should be performed continuously for at least 45 minutes.