Objectives To assess the effects of bariatric surgery in patients with obesity on dyspnea and to analyze the relationships between improvement of dyspnea after bariatric surgery and changes in pulmonary function, especially Expiratory Reserve Volume (ERV) which is the lung volume abnormality most frequently associated with obesity. Methods Forty-five patients (5 males/40 females, mean Body Mass Index = 46.2 ± 6.8 kg/m 2) were evaluated before and 6 to 12 months after bariatric surgery. Dyspnea was assessed by the modified Medical Research Council (mMRC) scale. Pulmonary function tests, arterial blood gases and six-minute walk test were performed. Laboratory parameters including C-Reac-tive Protein (CRP) were analyzed. Results Ninety percent of patients were dyspneic before surgery (mMRC scale ! 1) versus 59% after surgery (p<0.001). Mean mMRC score improved after bariatric surgery (1.5 ± 0.9 vs 0.7 ± 0.7, p<0.0001). Among patients with dyspnea before surgery (n = 38), a more marked increase in ERV after surgery was observed in patients with improvement of dyspnea compared to patients with no improvement of dyspnea (+0.17 ± 0.32 L vs +0.49 ± 0.35 L, p = 0.01). Multivariate analysis including age, variation of BMI, variation of CRP, variation of Total Lung Capacity and variation of ERV demonstraded that ERV was the only variable associated with improvement of the mMRc score after bariatric surgery (p = 0.04).