Ectopic production and secretion of hormones by a wide variety of tumors were initially recognized by signs and symptoms of excess circulating biologically active hormone. With the development of more sophisticated and sensitive techniques, it has become apparent that not all tumors secrete biologically active hormones. Some altered forms of polypeptide hormones may be in very high concentrations immunologically but be inactive biologically. On the other hand, polypeptide hormones may circulate at concentrations too low to induce clinical signs and symptoms. Consequently, new ectopic humoral syndromes have been recognized and the incidence of previously described syndromes has become considerably greater than previously recognized. Every hormone known to be normally secreted by endocrine organs or the placenta has been documented to be secreted ectopically by a wide variety of tumors. Moreover, several of those hormones may be used as biochemical markers of malignancy for both screening and monitoring of patients with documented or suspected tumors.