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Background: there is an 80% improvement glycemic control after gastric bypass in diabetic patients. Why some diabetic patients do not respond to surgery has yet to be explained.Methods: Seventeen obese Caucasian women (eight diabetic and nine euglycemic) were tested along with nine lean euglycemic control subjects. Tests were performed at three time points relative to surgery: 2-4 weeks before surgery, the week of surgery, and 3 months post-surgery. At each time point relative to surgery, an IV glucose tolerance test with minimal model analysis was performed, followed by a 150kcal Mixed Meal Tolerance Test (MMTT). There were 8 “responders” and 3 “non-responders” whose data was subsequently compared.Results: Insulin secretion in response to a meal challenge was markedly less in non-responders than in responders before surgery. 3 months post-surgery non-responders secreted as much insulin as responders. Post-surgery, GLP-1 secretion in response to a meal demonstrated no difference between responders and non-responders. Insulin sensitivity (ISI) improved in responders but not in non-responders. Conclusion: Pancreatic beta cells failure can not explain the results since insulin secretion is at least as robust in the non-responders as it was in responders. Our data support the hypothesis that “failure” of diabetes reversal is likely due to an inability to recover insulin sensitivity.