One-hundred micrograms corticotropin-releasing factor (CRF) was administered as an i.v. bolus to 6 women and one man patient with pituitary ACTH dependent Cushing's disease, one patient with ectopic ACTH secretion from a lung oat cell carcinoma, and 2 patients with hypercortisolism due to an unilateral adrenal adenoma and adrenal carcinoma. Though 4 out of 7 patients with pituitary ACTH-dependent Cushing's disease had normal basal ACTH levels, all 7 patients showed hyperresponsiveness of ACTH secretion after CRF administration compared to normal controls. Cortisol levels showed a similar pattern, with a significantly higher cortisol increase compared to normal controls. In contrast ACTH hypersecretion of ectopic origin could not be stimulated further by CRF administration, ACTH and cortisol remaining unchanged over the test period of 120 minutes. Suppressed ACTH levels below 5 pg/ml in the 2 patients with hypercortisolism of adrenal origin remained suppressed and cortisol levels did not change after CRF. We conclude that CRF administration is an effective tool in the differential diagnosis of Cushing's syndrome.