A woman with pituitary-dependent Cushing's disease remained hypercortisolemic after bilateral adrenalectomy. A search for an adrenal remnant by venous catheterization study suggested persistent cortisol-secreting tissue in the left adrenal bed. During ACTH stimulation, plasma corticosterone concentrations remained low and cortisol remained high, suggesting the cortisol was of exogenous origin. Cushingoid features resolved after confronting the patient with this evidence. Plasma corticosterone concentrations played a novel role in the diagnosis of factitiously induced Cushing's syndrome in this patient.