OBJECTIVETo characterize the magnitude and variance of the change of glucose and glucagon-like peptide-1 (GLP-1) concentrations, and to identify determinants of glucose control up to 2 years after gastric bypass (GBP).RESEARCH DESIGN AND METHODSGlucose and GLP-1 concentrations were measured during an oral glucose challenge before and 1, 12, and 24 months after GBP in 15 severely obese patients with type 2 diabetes.RESULTSGlucose area under the curve from 0 to 180 min (AUC0–180) started decreasing in magnitude (P < 0.05) 1 month after surgery. GLP-1 AUC0–180 increased in magnitude 1 month after GBP (P < 0.05), with increased variance only after 1 year (Pσ2 ≤ 0.001). GLP-1 AUC0–180 was positively associated with insulin AUC0–180 (P = 0.025).CONCLUSIONSThe increase in variance of GLP-1 at 1 and 2 years after GBP suggests mechanisms other than proximal gut bypass to explain the enhancement of GLP-1 secretion. The association between GLP-1 and insulin concentrations supports the idea that the incretins are involved in glucose control after GBP.