["There is no endocrine test which is completely reliable for the confirmation of Cushing's syndrome and in separation of the various aetiologies. We have tested the hypothesis that overnight dexamethasone pre-treatment should result in a better performance of the lysine-vasopressin (LVP) test in the diagnosis of Cushing's syndrome.", "We studied 61 subjects, including 25 pituitary-dependent and 9 pituitary independent Cushing's (7 adrenal tumour and 2 ectopic ACTH syndromes), 18 euadrenal controls, 4 depressed subjects, and 5 cushingoid patients. The subjects received 1 mg of dexamethasone orally at 2300 h and the following morning they were given 10 IU of lysine-vasopressin im.", 'Plasma cortisol (RIA) was measured at times -15, 0, 15, 30, 45, 60, 75, 90 and 120 minutes.', "The dexamethasone-modified LVP (Dx/LVP) test resulted in four patterns of cortisol response. The dexa sensitive pattern (positive suppression and negative response to LVP) was found in euadrenal subjects; the dexa insensitive pattern (negative suppression and positive response to LVP) was seen in Cushing's disease; a non-responsive pattern (negative suppression and negative response to LVP) was observed only in pituitary independent Cushing's; and an indeterminate pattern (positive suppression and positive response to LVP) was equivocal, being observed in 2 control subjects, 1 patient with Cushing's disease and 1 depressed patient. In separating control subjects from Cushing's syndromes the Dx/LVP test had 88.9% sensitivity, 100% specificity and 96.2% diagnostic accuracy; when the test was used to segregate Cushing's disease from control subjects we found 96.0% sensitivity, 100% specificity and 97.7% diagnostic accuracy. The performance variables for the Dx/LVP test in separating pituitary dependent from pituitary independent Cushing's were uniformly 100%. Depressed and cushingoid subjects did not differ from control subjects in their cortisol patterns during the test. Successful removal of the pituitary microadenoma in Cushing's disease was invariably followed by a reversal of the abnormal cortisol pattern (dexa insensitive) during the test to a dexa sensitive pattern indistinguishable from that of control subjects."] Does overnight dexamethasone pre-treatment improve the performance of the lysine-vasopressin test in the diagnosis of Cushing 's syndrome?