n patients with so-called functioning or syndromic pancreatic neuroendocrine tumors (PanNETs), specific biochemical tests should be requested in blood or 24-h urine samples obtained with or without provocative testing. Levels of circulating markers or urinary excreted products can be monitored and used for tumor follow-up. These tumors include gastrinomas, insulinomas, VIPomas, glucagonomas, GHRHomas, ACTHomas, PTHrPomas, somatostatinomas, PanNETs causing the carcinoid syndrome, and PanNETs secreting cholecystokinin (CCK), insulinlike growth factor II (IGF-II), renin, luteinizing hormone (LH), and erythropoietin