In this review, we evaluate radiological techniques currently used to localize gastroenteropancreatic (GEP) endocrine tumors. We also describe the visualization, using intravenous (IV) administration of two isotope-labeled somatostatin analogues (123I-Tyr3-octreotide and 111In-DTPA-octreotide) of islet-cell tumors in 25 patients and carcinoids in 39 patients. The primary tumor and previously unrecognized distant metastases were visualized in 20 of the 25 patients (80%) and in 37 of the 39 patients (95%). Parallel in vitro detection of somatostatin receptors on those tumors also visualized in vivo showed that ligand binding to the tumor in vivo represents binding to specific somatostatin receptors. The detection of somatostatin receptors on tumors in vivo predicted a good suppressive effect of octreotide on hormonal hypersecretion by these tumors. It is an easy, quick, and harmless procedure that is valuable in the localization of primary endocrine pancreatic tumors and their often radiologically and clinically unrecognized metastases. Future prospective controlled studies comparing this procedure with other radiological investigative techniques should demonstrate its sensitivity and specificity and determine the place of somatostatin receptor imaging in the localization of GEP endocrine tumors.