We retrospectively studied the records of 175 consecutive patients in whom OSAS had been diagnosed in our center. All patients underwent anthropometric evaluations and forced spirometry using a bell spirometer with a water seal. Diurnal arterial blood gas sampling while breathing room air was obtained from the radial artery. Polysomnography was performed and interpreted following standardized procedures. Patients with an apnea-hypopnea index (AHI) ≥ 10 received a diagnosis of OSAS. COPD was diagnosed in patients with FEV1 values < 80% of the predicted value and FEV1/FVC ratios < 70%. For analyzing the data, we first classified the patients into the following two groups: those with diurnal Paco2 ≥ 46 mm Hg (ie, hypercapnic OSAS [H-OSAS]); and those with Paco2 < 46 mm Hg (ie, normocapnic OSAS [N-OSAS]). The main characteristics for both groups were compared, using unpaired t tests and χ2 tests, when applicable. As a second step, correlations among diurnal Paco2 and spirometric parameters (ie, FEV1, FVC, and FEV1/FVC ratio), gasometric parameters (ie, Pao2, Paco2, and pH), polysomnographic parameters (ie, AHI), demographic parameters (ie, age), and anthropometric parameters (ie, body mass index [BMI]) were searched for all patients, using the Pearson correlation coefficient. Finally, multiple regression analysis was performed, introducing diurnal Paco2 as the dependent variable and those parameters that previously had been found to correlate with Paco2 using the Pearson correlation coefficient, as independent variables. The results were expressed as the mean ± SD, unless otherwise indicated.