Adrenocorticotropin (ACTH)-producing tumors of the pituitary gland usually, though not in all cases, initially show evidence of excess cortisol. The pathogenesis of these tumors--monoclonal versus polyclonal and intermediate lobe versus anterior lobe--continues to prompt debate. Important advances in the diagnostic methods (such as petrosal sinus sampling for ACTH and other hormones) are described. While the mainstay of therapy is still selective adenomectomy via the transsphenoidal approach, total hypophysectomy may occasionally be indicated.