This paper presents the results of the visualization of somatostatin (SS) receptor positive tumors in man after the i.v. administration of the SS analog Tyr3-octreotide coupled to 123I. It is an easy, quick and harmless procedure which allows imaging of primary and (often unexpected) secondary deposits and/or multiple localizations of the majority of endocrine pancreatic tumors, metastatic carcinoids and pituitary tumors, as well as of a multitude of tumors with neuroendocrine characteristics and well-differentiated brain tumors and meningiomas. In the case of hormone-secreting tumors a positive scan in most instances also predicts the subsequent successful therapy with octreotide.