Bilateral simultaneous inferior petrosal sinus sampling, in combination with CRH stimulation, is now able to confirm the diagnosis of pituitary-dependent Cushing's disease with near certainty. In expert hands, the procedure is straightforward and safe. As well as confirming the differential diagnosis, the test may aid surgical success, especially if no adenoma is apparent at transsphenoidal exploration. In this article, we review the technique and its interpretation and consider which patients should undergo the procedure.