Obesity causes changes in lung function, often the cause for the clinical manifestations presented by obese subjects. These changes are originated by the presence of excess fat tissue in the chest and the abdominal cavity that exerts a high pressure on the diaphragm and lungs which may lead to a reduction in lung volumes and a compromise in gas exchange. This article´s purpose is to characterize the changes in lung function resulting from the presence of a high body mass index, looking in particular for lung volumes and diffusion capacity for carbon monoxide.