Adrenocorticotropic hormone (ACTH), also known as corticotropin, is produced from corticotropes in the anterior lobe of the pituitary gland.  ACTH is released by corticotropin-releasing hormone stimulation by the hypothalamus in response to biological stress.  Identifying ACTH is useful in the classification of pituitary tumors and diseases.  Cushing’s disease derived from ACTH producing adenomas represents 10-15% of pituitary tumors.  By immunohistochemistry (IHC), these tumors are immunoreactive for ACTH, beta-Lipotropin, and beta-endorphin.  ACTH is also detected in a subset of growth hormone (GH), thyroid-stimulating hormone (TSH) and follicle-stimulating hormone (FSH) secreting adenomas.  With the exception for ACTH secreting adenomas, most functioning adenomas are usually macroadenomas; tumors larger than 10 mm.  A combination of ACTH IHC and reticulin staining also provides utility in the differential diagnosis between pituitary adenoma versus corticotroph hyperplasia.  Breakdown of acinar structure and ACTH immunoreactivity are suggestive of adenoma.