Radical surgery represents the only curative treatment for pancreatic neuroendocrine neoplasms (PanNEN). The aim of this study was to evaluate the postoperative onset of diabetes mellitus (DM) and/or pancreatic exocrine insufficiency (PEI) in surgically treated PanNEN. Consecutive PanNEN patients, without preoperative DM, who underwent partial pancreatic resection, were included. After a median follow-up of 72 months, overall 68/276 patients (24%) developed DM. Patients who developed DM were significantly older (p = 0.002) and they had a higher body mass index (BMI) (p < 0.0001) than those who did not; they were more frequently male (p = 0.017) and with nonfunctioning neoplasms (p = 0.019). BMI > 25 Kg/m2 was the only independent predictor of DM (p = 0.001). Overall, 118/276 patients (43%) developed a PEI, which was significantly more frequent after pancreaticoduodenectomy (p < 0.0001) and in patients with T3-T4 tumors (p = 0.001). Pancreaticoduodenectomy was the only independent predictor of PEI (p < 0.0001). Overall, 54 patients (20%) developed disease progression. Patients with and without DM had similar progression free survival (PFS), whereas patients without PEI had better five-year-PFS (p = 0.002), although this association was not confirmed in multivariate analysis. The risk of DM and PEI after surgery for PanNEN is relatively high but it does not affect PFS. BMI and pancreatic head resection are independent predictors of DM and PEI, respectively.