The association of chronic obstructive pulmonary disease (COPD) and sleep apnea syndrome (SAS), which are both frequent diseases, is likely to occur in a number of patients. We have prospectively investigated a large series (n = 265) of patients who were selected solely on the basis of a confirmed diagnosis of SAS (apnea + hypopnea index > 20/hr). An obstructive spirographic pattern, defined by an FEV1/VC ratio < or = 60%, was observed in 30 of 265 patients (11%). These patients (subgroup "overlap") were older (58 +/- 9) versus 53 +/- 10 yr, p = 0.01) than the remainder of the study population, and all were male patients. Body mass index (BMI) was identical in overlap patients to that in the remainder. Vital capacity and FEV1 were lower, by definition, in the overlap group. PaO2 was lower (66 +/- 10 versus 74 +/- 10 mm Hg, p < 0.001) and PaCO2 higher (42 +/- 6 versus 38 +/- 4 mm Hg, p < 0.001) in the overlap group. Hypoxemia (Pao2, < or = 65 mm Hg) was observed in 17 of 30 overlap patients and in 54 of 235 of the remainder. Hypercapnia (Paco2 > or = 45 mm Hg) was observed in 8 of 30 overlap patients and in 19 of 235 of the remainder. The pulmonary artery mean pressure (PAP) was higher in overlap patients both at rest (20 +/- 6 versus 15 +/- 5 mm Hg, p < 0.01) and during steady-state exercise (37 +/- 12 versus 29 +/- 10 mm Hg, p = 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)