We aimed to study whether the presence of obstructive sleep apnea syndrome (OSAS) in patients with chronic obstructive pulmonary disease (COPD) led to differences in clinical picture, gas exchange during awake and sleep states and mechanical ventilation, in comparison with patients with COPD alone. We enrolled 48 COPD patients. In 26 (54.1%), OSAS was ruled out (non-OSAS COPD group) by polysomnography, and in 22 (45.8%) associated OSAS was diagnosed (OSAS COPD group). Patients in the OSAS COPD group experienced greater daytime sleepiness and less dyspnea. Body mass index was not significantly difference. The OSAS COPD group had significantly lower daytime PaO2 (66.4 +/- 10.4 mmHg in the OSAS COPD group and 75.5 +/- 11.2 mmHg in the non-OSAS COPD group; p = 0.01); there were no differences in PaCO2.Pimax in the OSAS-COPD group was 70.6 +/- 23.8 cmH2O, a level that was significantly lower than in the non-OSAS COPD group (Pimax 90.5 +/- 26.1 cmH2O; p = 0.04). Patients in the non-OSAS COPD group experienced longer periods of REM sleep. Nighttime saturation parameters were significantly different in the group with OSAS. We conclude that patients with both OSAS and COPD experience greater oximetric changes than those without OSAS, during both sleep and awake states. The deterioration of respiratory muscle pressures in such patients may play an important role in the changes. The groups also present differences in the intensity of some symptoms, such as degree of daytime sleepiness and dyspnea.