High affinity somatostatin receptors (SS-R) have been identified in membrane homogenates or tissue sections from several hundred human tumors. SS-R were found in most tumors originating from SS target tissues, i.e. GH and TSH producing pituitary tumors, endocrine gastroenteropancreatic (GEP) tumors and brain tumors. SS-R were also expressed in several tumors originating from various other tissues, i.e. breast and small cell lung carcinomas, some colorectal cancers, and medullary thyroid carcinomas. In general, most of the SS-R positive tumors are well differentiated and/or have neuroendocrine features. Among endocrine GEP tumors, more than 80% of carcinoids and 70-100% of islet cell carcinomas have a high density of SS-R. All their metastases are SS-R positive. Undifferentiated, atypical carcinoids are usually SS-R negative. SS-R are functional and mediate hormone secretion inhibition. SS-R positive tumors and metastases can easily be localised non-invasively in vivo by scanning techniques after 123I-SS analog injection.