Corticotropin-releasing hormone, a 41-amino-acid peptide, is an important hypothalamic factor regulating the pituitary secretion of adrenocorticotropic hormone (ACTH). This hormone, while rarely involved in the etiology of Cushing's syndrome, seems to play a major role in the pathogenesis of non-Cushing hypercortisolemic states. The administration of synthetic human and ovine CRH has been found useful in the differential diagnosis of Cushing's syndrome and in the preoperative localization of intrasellar ACTH-secreting tumors.