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Background: Chronic obstructive pulmonary disease (COPD) patients are very sensitive to changes in pulmonary mechanics and central ventilation control during sleep and may develop significant gas exchange alterations with increased hypoxemia and hypercapnia. Oxygen therapy improves nocturnal desaturation but can worsen hypoventilation. Objectives: To analyze the prevalence of nocturnal hypoventilation (NHV) in hypercapnic COPD patients and to determine predictive factors for this phenomenon. Methods: This was a prospective multicenter study which enrolled 80 clinically stable COPD patients with hypercapnic respiratory failure who fulfilled the conventional criteria for long-term oxygen therapy (LTOT). All patients had undergone pulmonary function testing, blood gas analysis, and respiratory polygraphy. Arterial blood gas samples were obtained while patients were awake and during sleep. NHV was considered when an increase in PaCO2 >10 mm Hg was observed in any nocturnal arterial blood gas sample as compared to the awake levels. Results: Seventeen patients (21%) developed NHV. NHV was associated with the values of BMI, hemoglobin, hematocrits, DLCO, and PaO2 reached after oxygen administration. In the logistic regression analysis BMI (OR 1.26, 95% CI 1.068–1.481; p = 0.006) and the diurnal increase of PaO2 after O2 (OR 0.89, 95% CI 0.807–0.972; p = 0.010) were the variables that best discriminated with a sensitivity of 82% and a specificity of 78%. Conclusions: NHV is a relatively common finding in stable hypercapnic COPD patients undergoing LTOT and it is related to a higher BMI and lower PaO2 after oxygen administration.