Type 2 diabetes mellitus (T2DM) has a very strong association with obesity. The aim of our study was to analyze the effects of Roux-en-Y gastric bypass (RYGB) surgery on the glucose metabolism in morbidly obese patients with T2DM. Morbidly obese patients (n = 117) with T2DM underwent measurements of fasting serum glucose and glycosylated hemoglobin (HbA1C) at baseline, 6 months, and 12 months after laparoscopic RYGB surgery. Logistic regression was used in both univariate and multivariate modeling to identify independent variables associated with complete resolution of T2DM. Twelve months after surgery, fasting plasma glucose decreased from a preoperative mean of 164 +/- 55 mg/dL to 101 +/- 38 mg/dL (P = .001) and HbA1C decreased from a preoperative mean of 7.7% +/- 1.5% to 6.0% +/- 1.1% (P = .001). Resolution of T2DM was achieved in 72 patients (74%). All of the remaining 25 patients decreased the daily medication requirements. On univariate analysis, preoperative variables associated with resolution of T2DM were waist circumference, HbA1C, and absence of insulin treatment. Waist circumference (odds ratio 2.4; 95% confidence interval 1.4-4.1; P = .001) and treatment without insulin (odds ratio 42.2; 95% confidence interval 4.3-417.3; P = .002) remained significant predictors of T2DM resolution in the multivariate logistic regression model after adjusting for covariates. Laparoscopic RYGBP resulted in significant resolution of T2DM. Peripheral fat distribution (smaller waist circumference) and absence of insulin treatment were independent and significant predictors of complete resolution of T2DM.