Theoretically urinary free corticosteroid excretion should be affected by renal function and this would make it a less sensitive index of hypercortisolaemia. In 28 consecutive urine samples there was a clear relationship (r = 0.83; P less than 0.001) over a range of creatinine clearances 0.3-200 ml/min. Although an allowance could be made for renal function this would not necessarily improve the discrimination of normal from abnormal. Until data comparing corrected to uncorrected urinary free corticosteroid excretion become available, we recommend a short dexamethasone test as the initial investigation in patients with suspected hypercortisolaemia and abnormal plasma creatinine concentrations.