BACKGROUND: Nocturnal hypoxemia occurs in patients with chronic obstructive pulmonary disease(COPD) and the detection and treatment of nocturnal hypoxemia should be part of the management of COPD patients. We performed this study to evaluate the factors influencing to sleep related arterial oxygen desaturation(SaO2) in patients with COPD. METHODS: Resting and exercise cardiopulmonary function test, polysomnography, and SaO2 during resting, exercise and sleep were measured in 12 patients with COPD. RESULTS: The SaO2 fell twice as much during sleep as during maximal exercise(13.1 9.3% fall in nocturnal SaO2 vs. 6.4 3.3%, p<0.05). Fall in nocturnal SaO2 was well correlated with mean exercise SaO2(r=-0.78, p<0.05), minimum exercise SaO2(r=-0.90, p<0.01), and resting SaO2(r=-0.82, p<0.05). Lowest sleep SaO2 was well correlated with mean exercise SaO2(r=0.80, p<0.05), lowest exercise SaO2(r=0.90, p<0.01), and resting SaO2(r=0.84, p<0.05). CONCLUSION: Resting and exercise SaO2 was well correlated with nocturnal SaO2, but exercise study add no additional information to predicting the nocturnal oxygen desaturation in patients with COPD.