["The diagnosis of Cushing's syndrome remains a major challenge in clinical endocrinology. Various screening tests are commonly used to support a biochemical diagnosis in the context of clinical suspicion. The aim of this study was to compare the sensitivity in the diagnosis of Cushing's syndrome of a single in-patient sleeping midnight cortisol to a standard 48-hour in-patient low-dose dexamethasone suppression test (LDDST) during the same admission.", "A retrospective analysis was performed on 150 patients investigated in our department between the years 1970 and 1994 with a confirmed diagnosis of Cushing's syndrome.", "One hundred and fifty patients with a diagnosis of Cushing's syndrome were analysed: 110 with Cushing's disease; 12 with tumours with ectopic ACTH secretion; 8 with ACTH dependent Cushing's syndrome of so far undetermined origin; 17 with cortisol secreting adrenal tumours; 3 with adrenocortical nodular hyperplasia. Twenty normal volunteers and nine patients with non-endocrine conditions were also investigated as controls.", 'Plasma cortisol was measured by radioimmunoassay (RIA) in the 122 patients presenting after 1980, and by fluorimetry prior to this date.', "In all the control subjects the sleeping midnight cortisol was < 50 nmol/l, below the lowest standard of the routine in-house RIA. In every patient with Cushing's syndrome the sleeping midnight cortisol was detectable with a value greater than 50 nmol/l, with a range of 70-2000 nmol/l. In contrast, in three cases, all of whom had proven Cushing's disease on histology, there was uncharacteristic complete suppression of plasma cortisol to < 50 nmol/l following the LDDST."] Does a single sleeping midnight cortisol have 100 % sensitivity for the diagnosis of Cushing 's syndrome?