Background: Obesity is a global and growing public health problem. Bariatric surgery (BS) is indicated in patients with morbid obesity. To our knowledge, theeffects of morbid obesity and BS on ventilation/perfusion (VA/Q) ratio distributions using the multiple inert gaselimination technique have never before beenexplored. METHODS: We compared respiratory and inert gas (VA/Q ratio distributions) pulmonary gas exchange, breathing both ambient air and 100% oxygen, in 19 morbidly obese women (BMI, 45 kg/m2), both before and 1 year after BS, and ineight normal-weight, never smoker, agematched, healthy women. RESULTS: Before BS, morbidly obese individuals had reduced arterial Po2 (76±2 mm Hg) and an increased alveolar-arterial Po2 difference (27±2 mm Hg) caused by small amounts of shunt (4.3%±1.1% of cardiac output), along with abnormally broadly unimodal blood flow dispersion (0.83±0.06). During 100% oxygen breathing, shunt increased twofold in parallel with a reduction of blood flow to low VA/Q units, suggesting the development of reabsorption atelectasis without reversion of hypoxic pulmonary vasoconstriction. After BS, body weight was reduced significantly (BMI, 31 kg/m 2), and pulmonary gasexchange abnormalities were decreased. CONCLUSIONS: Morbid obesity is associated with mild to moderate shunt and VA/Q imbalance. These abnormalities are reduced after BS.