The mechanism by which incretins and their effect on insulin secretion increase markedly following gastric bypass (GBP) surgery is not fully elucidated. We hypothesized that a decrease in the activity of dipeptidyl peptidase-4 (DPP-4), the enzyme which inactivates incretins, may explain the rise in incretin levels post-GBP. Fasting plasma DPP-4 activity was measured after 10-kg equivalent weight loss by GBP (n = 16) or by caloric restriction (CR,n = 14) in obese patients with type 2 diabetes. DPP-4 activity decreased after GBP by 11.6% (p = 0.01), but not after CR. The increased peak glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) response to oral glucose after GBP did not correlate with DPP-4 activity. The decrease in fasting plasma DPP-4 activity after GBP occurred by a mechanism independent of weight loss and did not relate to change in incretin concentrations. Whether the change in DPP-4 activity contributes to improved diabetes control after GBP remains therefore to be determined.