Radiolabeled somatostatin analogs are potentially of considerable value in both the localization and treatment of somatostatin receptor-bearing tumors. Whole body 123I-labeled Tyr3-octreotide scintigraphy is capable of detecting and localizing primary tumors and metastatic disease that may not be detectable by other methods. Although it is possible that this technique may be applicable to a wide variety of neoplasms, 123I-Tyr3-octreotide scans appear particularly useful for imaging various gut neuroendocrine tumors, meningiomas, and paragangliomas. This primarily reflects the level of expression of somatostatin receptors by these lesions. In addition to whole body scintigraphy, intraoperative localization by receptor identification utilizing a hand-held gamma detector probe may prove to be an effective means of identifying spread and micrometastasis of somatostatin receptor-bearing tumors. Aside from identification and topographic localization of disease, the detection of an 123I-Tyr3-octreotide labeled tumor may be utilized as in vivo assay for the presence of somatostatin receptors. Such observations may prove useful in predicting the susceptibility of an individual tumor to octreotide therapy. Additionally, it may prove possible to deliver a therapeutic dose of radiation to receptor-bearing tumors by the administration of specific radiolabeled somatostatin analogs.