&NA;A new method of intraoperative measurement of adrenocorticotropic hormone for the localization of microadenomas in Cushing's disease is described and, in 21 cases, compared with preoperative bilateral sampling of the inferior petrosal sinus. By intraoperative measurement of adrenocorticotropic hormone in peripituitary venous blood, it was possible to determine a gradient for adrenocorticotropic hormone in 14 of 19 patients with surgically proven adenomas. In 4 of 6 patients in whom a false positive bilateral sample from the inferior petrosal sinus directed the surgeon to the wrong side of the pituitary, the localization could be correctly determined by intraoperative radioimmunoassay of adrenocorticotropic hormone. (Neurosurgery24:201‐205, 1989)