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After major surgery, analgesia with opioids can cause obstructive apnea and intermittent hypoxemia, probably from loss of upper airway control. Since this resembles the obstructive sleep apnea syndrome, we tested the possibility that nasal continuous positive airway pressure (nCPAP) would reduce episodes of reduced oxygen saturation and sleep disruption. Because oxygen therapy is frequent after surgery, we also assessed the effect of oxygen on sleep disruption.We recruited 48 patients about to have major abdominal surgery. We present data for 34 patients: 27 who received patient-controlled intravenous morphine and 7 who received epidural opioid. Treatment was randomized to either nCPAP or conventional therapy with an oxygen mask. Alternate periods of administration of air and 35% oxygen were used in both groups. If the oxygen saturation as measured by pulse oximetry was consistently <90% on air, the patient was withdrawn from the study. We measured sleep, arousals, oxygenation, episodes of desaturation, and disturbances of respiration. Values are given as median and quartiles.The median proportion of time awake was 65% (45–79%) among control patients and 74% (55–87%) among those undergoing nCPAP. Oxygen administration did not affect the sleep pattern. The median frequency of arousals per hour of sleep was very similar in each group: during air breathing from nCPAP, 125 (76–187), and during air breathing by mask, 116 (84–187). Oxygen therapy had no effect. Oxygenation and hypoxemic events were not improved by nCPAP. Oxygen therapy improved oxygenation and reduced but did not eliminate episodes of desaturation.Nasal CPAP does not improve sleep and oxygenation or reduce hypoxemic events in the first night after major abdominal surgery.