Evidence for and against the hypothesis that GH-, PRL-, and ACTH-secreting tumors arise as the result of hypothalamic dysregulation has been presented. For each of these types of tumor it appears that similar proportions--perhaps 80 to 90 per cent arise de novo within the pituitary. Although it is quite possible, it is not yet proven that the remaining 10 to 20 per cent of cases arise as a result of underlying hypothalamic dysregulation. The number of cases of TSH- and gonadotropin-producing tumors is too small to allow this type of analysis, but there is no completing reason to suspect that they differ in etiology from these other tumor types. The subsets of gonadotropin- and TSH-secreting tumors that arise as a result of primary target organ failure are discussed in "Thyrotropin-secreting Pituitary Tumors" and "Gonadotroph Cell Pituitary Tumors," respectively.