Obese patients frequently present with respiratory symptoms, including dyspnea, explained in part by the fact that obese individuals tend to breathe rapidly and shallowly as an adaptation to the increase in total respiratory work and resistance caused by obesity. However, the clinician is frequently asked to determine whether these findings are simply caused by obesity alone or whether they represent a respiratory illness. Accurate interpretation of spirometry performed on obese patients requires an understanding of the effect of obesity severity and distribution on lung volumes and airway size. In mild obesity, results of spirometry might be normal or might suggest a restrictive process, with a symmetric reduction in FEV1 and forced vital capacity (FVC). Some investigators have observed a disproportionate reduction in FVC with obesity, demonstrating that body mass index (in kilograms per square meter) is significantly associated with the FEV1/FVC ratio (P< .01). 3 In contrast, individuals with extreme obesity can demonstrate airflow limitation on spirometry. In one study of subjects with a body