Breathing at low lung volumes alters both respiratory mechanics and gas exchange. Because airflow resistance increases at lung volumes below FRC, especially as lung volume approaches RV, one might expect evidence of airflow obstruction in patients who are obese, even in the absence of airway disease. However, the data are conflicting; some investigators report an increased prevalence of airflow obstruction in individuals who are obese, and others fail to find such a relationship.6,10 At low lung volumes, there is also a tendency for the small peripheral airways to collapse, leading to decreased ventilation of the lung bases, ventilation-perfusion mismatch, and hypoxemia. For these reasons, oxygenation, as reflected by both the Pao2 and the alveolar-arterial Po2 difference, is related to FRC: As FRC diminishes, the alveolar-arterial Po2 difference increases and the Pao2 declines, and improvements in oxygenation are correlated with increases in FRC after weight loss. Thus, in our patient, the borderline normal TLC, marked reduction in FRC, and mild reduction in awake, resting oxygen saturation are all typical of the physiologic changes accompanying obesity.