Resolution of Type-2 diabetes mellitus (DM) after weight loss surgery is well documented, but the mechanism is elusive. We evaluated the glucose-insulin metabolism of patients undergoing a Roux-en-Y gastric bypass (RYGB) using the intravenous glucose tolerance test (IVGTT) and compared it with patients who underwent laparoscopic adjustable gastric band (AB) placement. Thirty-one female patients (age range, 20 to 50 years; body mass index, 47.2 kg/m2) underwent RYGB. Nine female patients underwent AB placement and served as control subjects. All patients underwent IVGTT at baseline and 1 month and 6 months after surgery. Thirteen patients undergoing RYGB and one patient undergoing AB exhibited impaired glucose tolerance defined by the American Diabetes Association. By 6 months post surgery, diabetes was resolved in all but one patient undergoing RYGB and none of the patients undergoing AB. Patients with diabetes undergoing demonstrated increased insulin secretion and beta-cell responsiveness 1 month after surgery and continued this trend up to 6 months, whereas none of the patients undergoing AB had changes in beta-cell function. Both patients undergoing RYGB and those undergoing AB demonstrated significant weight loss (34.6 and 35.0 kg/m2, respectively) and improved insulin sensitivity at 6 months. RYGB ameliorates DM resolution in two phases: 1) early augmentation of beta cell function at 1 month; and 2) attenuation of peripheral insulin resistance at 6 months. Patients undergoing AB only exhibited reduction in peripheral insulin resistance at 6 months but no changes in insulin secretion.