Contest-&-Objective We compared the incidence of hypoglycemia after Roux-en-Y-Gastric-Bypass (RYGB) versus Sleeve Gastrectomy (SG). Design-Setting-and- Main Outcome Measure Randomized, open-label trial conducted at the outpatient obesity-clinic in a university-hospital in Rome, Italy. The primary aim was the incidence of reactive hypoglycemia (<3.1 mmol/l after 75g-oral-glucose-load) at 1 year after surgery (intention-to-treat-analysis, carried-forward for missing data). Secondary aims were hypoglycemia under everyday-life conditions, insulin-sensitivity and insulin-secretion, lipid profile. Results Of 175 eligible patients, 120 were randomized 1:1 to RYGB or SG; 117 (93%) completed the 12-months follow-up. Hypoglycemia during oral-glucose-tolerance testing was detected in 14% and 29% of SG and RYGB patients (P=0.079), with the effect of treatment in multivariate analysis significant at P=0.018. Daily hypoglycemic episodes during continuous-glucose-monitoring did not differ between groups (P=0.75). Four out of 59 RYGB-subjects (6.8%) had 1-3 hospitalizations for symptomatic-hypoglycemia vs. 0 in SG. The static β-cell glucose-sensitivity-index, evaluated by minimal model, increased after both treatments (from 50.99±33.68∙10⁹min^–1 to 120.43±98.90∙10⁹min^–1, P<0.001), but the dynamic β-cell glucose-sensitivity-index increased significantly in SG (from 323.93±480.87∙10⁹ to 933.32±1063.86∙10⁹min^–1, P=0.008) and decreased in RYGB (from 645.69±733.21∙10⁹ to 414.90±469.87∙10⁹ min^–1; P=0.004 for time×treatment interaction). Whole-body insulin-sensitivity increased about 10-fold in both groups. The higher the BMI reduction, the higher was the number of hypoglycemic episodes. Conclusions We show that reactive-hypoglycemia is no less common after SG and this is not a safer option than RYGB, but RYGB is associated with more severe hypoglycemic episodes. This is likely due to the lack of improvement of β-cell sensitivity to changes in circulating glucose after RYGB, which determines an inappropriately high insulin-secretion in the face of a dramatically improved peripheral insulin-sensitivity.