Overproduction of corticotropin by the pituitary gland or extrapituitary tumors leads to hypercortisolism or Cushing syndrome. Diagnosis of suspected Cushing syndrome involves 3 major steps: confirmation of hypercortisolism, differentiation between corticotropin-independent and corticotropin-dependent causes of Cushing syndrome, and distinction between pituitary and ectopic corticotropin production. A definitive diagnosis of ectopic corticotropin secretion requires stringent criteria, including reversal of the clinical picture after resection of the tumor and/or demonstration of corticotropin immunohistochemical staining within the tumor tissue.