3. Either MRI confirmation of pituitary adenoma > 6 mm, or inferior petrosal sinus gradient >3 after CRH stimulation for those patients with a tumor less than or equal to 6 mm*. For patients who have had prior pituitary surgery, histopathology confirming an ACTH staining adenoma *If IPSS had previously been performed without CRH (e.g. with DDAVP), then a central to peripheral pre-stimulation gradient > 2 is required. If IPSS had not previously been performed, IPSS with CRH stimulation is required.