By the term "paraneoplastic syndrome" is meant a constellation of symptoms and findings that can be traced back to a tumor, but which are not caused by local or metastatic mechanisms. Such paraneoplastic syndromes can suggest the diagnosis of a tumor, or serve to chart the course of known tumor disease. When the pathophysiology is known, the mediators (hormones) may replace the clinical syndrome as useful parameters. With respect to the pathobiology of tumors, an analysis of paraneoplastic syndromes would seem to be highly promising, and might possibly form the basis for the development of new therapeutic concepts.