Biologic and radiologic investigations in the diagnosis of Cushing syndrome are evaluated. A two-step diagnostic procedure is propounded. The combination of absence of overnight suppression of plasma cortisol by dexamethasone and elevation of urinary free cortisol and midnight plasma cortisol levels establishes the diagnosis of Cushing syndrome in most patients. The next step is to determine the precise cause of Cushing syndrome by baseline plasma ACTH assays, the high-dose dexamethasone test and the visualization of the adrenals by iodocholesterol scintigraphy and/or computed tomography.