Urinary free cortisol excretion has been measured in a group of 40 patients with Cushing's syndrome and in 34 patients in whom this diagnosis was suspected on clinical grounds, but in whom it was later disproved.Measurements were made in the basal state, during dexamethasone suppression, and after metyrapone (when free cortisol and 11-deoxycortisol were measured together) and the results compared with those of 17 oxogenic steroid measurements.In the basal state no normal or‘suspected’patient excreted more than 106μg/24 h of free cortisol while the lowest value seen in any patient with Cushing's syndrome was 110μg/24 h. In 25 per cent of the patients with hyperadrenalism 17 OGS excretion was within the normal range. Three patients with ectopic ACTH produc tion all excreted more than 1000μg of free cortisol in 24 h and in two of the three 17 OGS excretion was greater than 100 mg/h.During low-dose dexamethasone suppression (2 mg daily) free cortisol excretion fell to very low levels in normal and suspected subjects but failed to do so in patients with Cushing's syndrome, although a few patients did show suppression almost to within the normal range. In no patient with Cushing's syndrome did 17 OGS excretion fall to less than 5 mg/24 h. However, it also failed to do this in 43 per cent of the‘suspected’subjects.High-dose dexamethasone suppression (8 mg/day) failed to provide a clear differentiation between patients with pituitary-dependent disease and those with autonomous adrenal lesions or ACTH-producing tumours in terms of change in either free cortisol or 17 OGS excretion.On metyrapone testing 96 per cent of pituitary-dependent patients showed a definite rise in 17 OGS excretion. We have not yet seen such a response in a patient with a non-pituitary-dependent lesion. Measurement of free F+S excret
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