In a rapid screening test for Cushing's syndrome, plasma cortisol was measured by a simple fluorometric method. We have verified the value of the overnight suppression test as a screening test for hyperadrenocorticism. An 8 AM plasma cortisol level of less than 10μg/ 100 ml after a single dose of 1 mg of dexamethasone was given orally the previous night at 11 PM essentially rules out Cushing's syndrome. Emotional or psychiatric disorders and acute illnesses may result in incomplete suppression. Oral contraceptive agents do not ordinarily result in a false-positive test. Patients who do not respond normally to overnight suppression with dexamethasone warrant further study.