The factors that contribute to the development of chronic cor pulmonale (CCP) in sleep apnea syndrome (SAS) are being continuously reviewed, as well as the role played by the coexistence of chronic obstructive pulmonary disease (COPD). Right ventricular function was evaluated in 20 SAS patients, 10 of whom presented associated (COPD). In all of them the following tests were performed while fasting: blood gasometry, spirometry, body mass index and isotopic ventriculography which included the determination of righ ventricular ejection fraction (RVEF). The RVEF of the group presenting COPD (mean +/- DS) (0.43 +/- 0.07) was not significantly different from the group without COPD (0.46 +/- 0.09). When the patients were regrouped according to the presence of hypoxemia during the day (paO2 less than 70 mmHg) a significant difference was evidenced between the mean value of the RVEF group with hypoxemia (n = 10) (0.40 +/- 0.02) and that of the group with normoxemia (n = 10) (0.50 +/- 0.09) (p less than 0.05). There were no statistically significant differences between the degree of obstruction and the BMI in either group. These results suggest that the worsening of RVF is more frequent in SAS patients who present daytime hypoxemia.