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Oxygen therapy for patients with sleep apnea-hypopnea syndrome (SAHS) usually causes significant side effects. The aim of this study was to assess the effect of short-term nocturnal oxygen therapy in patients with SAHS and chronic obstructive pulmonary disease. Ten patients with diagnoses of SAHS were enrolled. The patients' mean age was 63 (10) years, mean apnea-hypopnea index (AHI) was 58 +/- 17, mean FVC was 59 +/- 8% of reference and mean FEV1 was 40 +/- 14% of reference. Using a random, single blind design, two polysomnographic studies were performed on two consecutive nights. Oxygen was administered on one night at a mean flow rate of 1.3 +/- 04 l/min and on the other night air was administered at the same rate. Arterial blood gases were analyzed at the end of each study. Oxygen administration improved nocturnal hypoxia and reduced the AHI, which was 40 +/- 20 with oxygen and 58 +/- 17 with air (p < 0.005). Improvement was achieved at the expense of a reduction in the number of hypopneic episodes. No significant differences were observed in apneic episodes and only a slight increase in the duration of hypopneic episodes was observed (21 +/- 7 s with air and 27 +/- 8 s with oxygen [p < 0.01]). Neither quality of sleep nor heart rate changed. Slight respiratory acidosis was observed in 50% of the patients. In conclusion, nocturnal oxygen administration in patients with SAHS and COPD improved nocturnal hypoxia and reduced the total number of respiratory events. However, in these patients oxygen should be administered with care, even when the rate of flow is low, given the tendency for pCO2 and respiratory acidosis to increase.