The availability of CRH for clinical use and the data so far generated by clinical investigators interested in its application to the problem of Cushing's syndrome have resulted in a new approach to the differential diagnosis of Cushing's syndrome (Fig. 2). Once the diagnosis of Cushing's syndrome is made with certainty, an initial separation into ACTH-dependent versus ACTH-independent categories is made on the basis of the plasma ACTH response to CRH. Plasma ACTH levels in excess of 10 pg/mL imply an ACTH-dependent process. Patients with an ACTH-dependent process are then divided into eutopic versus ectopic sources on the basis of the central to peripheral gradient of ACTH measured in the inferior petrosal sinuses simultaneously 3 to 5 minutes after the administration of CRH, 1 microgram/kg. The average post-CRH gradient in patients with eutopic ACTH-dependent Cushing's syndrome is 50. The lower bound is 3.3. Ectopic sources have a gradient of less than 2. Patients with ACTH-independent Cushing's syndrome can be confidently evaluated with imaging techniques, such as CT or MRI, looking for adrenal neoplasm or the "normal to small" glands characteristic of micronodular adrenal dysplasia.