Basal values of the urinary excretion of 17-oxogenic steroids and serum levels of cortisol were not satisfactory in the differentiation of 'suspected' subjects from patients with true Cushing's syndrome. With an RIA method for serum cortisol determination, the overnight dexamethasone suppression test provided the most reliable single test in establishing adrenocortical hyperfunction. Thirty-five normal subjects, fifty-nine obese patients, thirteen 'suspected' patients, and thirteen patients with disease states other than Cushing's syndrome had suppressed values below 4.0 microgram/100 ml. None of the ten patients with Cushing's syndrome had a cortisol concentration less than 16.3 microgram/100 ml. Adrenal gland scintigraphy after radiocholesterol injection is a more valuable tool than the metyrapone test and the high-dose dexamethasone suppression test in the localization and differential diagnosis of adrenocortical lesions causing Cushing's syndrome. It obviates the need for angiographic procedure in the localization of adenomas. It is a reliable technique for identifying functioning adrenal remnants. We therefore propose a schedule for studying patients with suspected adrenocortical hyperfunction.