Rationale: Although arterial oxygen concentration decreases in obese subjects, the mechanism for this remains to be determined. Objectives: The blood gas level in each pulmonary vein was measured in supine subjects with diverse BMI values, to determine whether there was a regional insufficiency in gas exchange depending on the subject's BMI. Methods: Forty subjects with normal cardiopulmonary function who underwent a catheter ablation for atrial fibrillation were included. Before delivering any radiofrequency energy application, blood samples were obtained from each of the 4 pulmonary veins during physiological breathing in supine position in order to measure the PO₂ and PCO₂ values. Spirometry and ventilation/perfusion lung scintigraphy were also performed. Measurements and Main Results: The PO₂ value was significantly higher in superior veins than inferior veins (91.8 ± 13.5, 70.8 ± 16.3, 92.2 ± 11.1, and 73.6 ± 13.7 mmHg, in the left superior, left inferior, right superior and right inferior pulmonary vein, respectively). There was a significant inverse relationship between the PO₂ and PCO₂ values. Neither the spirometry nor lung scintigraphy could detect any remarkable findings corresponding to the low PO₂ values. Among the various clinical characteristics, only the BMI was significantly associated with the decreased PO₂ value in the inferior veins. Conclusions: Hypoxia in obese subjects in supine position is thus considered to be primarily caused by insufficient gas exchange in the regions of lung linked to the inferior PVs. The inverse relationship between the BMI and PO₂ value in the inferior pulmonary veins suggests a possible subclinical manifestation of obesity-related respiratory insufficiency.