Introduction Surgical treatment of obesity has not only resulted in weight loss, but also the control of associated diseases in the postoperative period. The aim of this study was to determine the post-op response of type 2 diabetes mellitus (DM2) to gastric bypass. Methods A historical cohort study was performed involving all the diabetic, morbid obese patients who underwent gastric bypass during the period of May 2000 to October 2006 at the Centro da Obesidade Morbida. Patient records were reviewed with regard to sex, age, anthropometric measurements, glycemia, glycosylated hemoglobin, pre-op insulin, and DM2 outcome. Results A total of 125 diabetic, morbid obese patients were operated; they had a mean age of 42.6+/-9.7 years and body mass index of 49+/-8.3 kg/m(2). Of these patients, 63 (50.4%) required medication for control of DM2, and the rest managed only with diet. Of these 63 patients, 43 (68.2%) achieved control of DM2 and were discharged without the need for antidiabetic medication, and 20 (31.7%) were discharged still on medication. One month after surgery, seven more patients discontinued medication for DM2, and 18 months after surgery, 97.6% of the patients showed control of DM2. Conclusion Gastric bypass is effective in controlling DM2 in morbid obese patients.