Gastric bariatric operation has been advocated to ameliorate adult-onset diabetes mellitus in the morbidly obese patient. However, there are few data to support this. In this clinical study we report the cases of 23 insulin-treated, morbidly obese, adult-onset diabetes (type II) patients who underwent gastric bariatric operations for weight reduction. Preoperative insulin requirements ranged from 10 to 230 U/day (mean 74 U/day). After operation, 14 patients discontinued insulin, and seven patients decreased their insulin requirement by 72% (100 to 28 U/day). Two patients were lost to follow-up. Six patients were admitted to the Clinical Research Unit before and after operation to evaluate the effects of surgically induced weight loss on fasting blood glucose levels, hemoglobin A1C insulin resistance, and glucose tolerance. Weight loss at the time of postoperative studies ranged between 22.5 and 49.1 kg (mean 30.6 kg). Fasting blood glucose levels decreased 48% (mean 321 mg/dl before operation to 166 mg/dl after operation). Hemoglobin A1C levels improved from a mean of 11.8% to 7.9%. Insulin resistance assessed in four patients by measurement of glucose clearance during insulin and glucose infusion improved from a mean of 77 ml/min before operation to 228 ml/min after operation. Glucose tolerance was quantitated by determination of glucose disappearance during intravenous glucose tolerance test. The mean glucose disappearance improved from 0.176% to 0.385%/min following operation. This study provides clinical and laboratory evidence indicating significant improvement of insulin-treated diabetes mellitus in morbidly obese patients who have lost weight following gastric bariatric operation.