Not so many years ago the general concept of tumors was that they are collections of cells which have cut themselves off from all functional activity, living in a purely parasitic way. It is known now that this is not invariably true and that a considerable degree of functional activity may be retained by the cells of certain tumors, especially those of highly differentiated type. In the case of neoplasms of the endocrine glands such a persistence of function has long been recognized, probably because its manifestations are more striking, involving tissues and functions far removed from the tumor site itself. Many examples of functional endocrine tumors are now adducible as producing well established clinical syndromes. Among these may be mentioned adenoma of the eosinophilic, basophilic or chromophobic cells of the anterior lobe of the pituitary and certain tumors of the thyroid, parathyroid and adrenal (both cortex and medulla) glands