Background: Asthma and sleep apnea syndrome (SAS) are frequently reported in obese patients. The authors determined the prevalence of asthma and SAS in morbidly obese patients and the effect of biliopancreatic diversion with duodenal switch (BPD-DS) on these conditions. Methods: 398 patients were evaluated for bariatric surgery in a university-affiliated tertiary care center. All patients completed a written questionnaire on asthma and SAS before BPD-DS. In addition, 139 patients also completed a questionnaire on their general health status, including asthma and SAS, 2 years after the procedure. Results: For the cohort of 398 patients, the prevalence of self-reported asthma was 30.4% and that of SAS, 32.2%. No significant association was found between asthma and SAS diagnosis (P =0.10). Significant relationships were observed between the diagnosis of asthma and age, hip circumference, waist/hip ratio, weight and BMI of the patients as well as between a diagnosis of SAS and gender, waist circumference, hip circumference, waist/hip ratio, weight and BMI. 2 years after surgery (mean BMI was reduced from 51.4 to 30.5 kg/m2), asthma was reported improved in 79.3% of patients and SAS was improved in all but one with this condition; among 29 SAS patients using CPAP before surgery, only 4 were still using this treatment after 2 years. Conclusion: The prevalence of asthma and SAS is high in the morbidly obese population and is associated with markers of obesity. We found no association between the diagnosis of asthma and SAS diagnosis in this population. BPD-DS improved self-reported severity of asthma and SAS symptoms.