This paper summarizes the original clinical experience of the authors concerning differential diagnostics of ACTH-dependent hypercortisolism. A total of 8 patients were available for the estimation of the potential of such diagnostics with the use of the high-dose dexamethasone suppression test, pituitary MRI, and selective blood sampling from the inferior petrosal sinuses for the determination of the ACTH concentration gradient between central and peripheral compartments. It turned out that neither the high-dose dexamethasone suppression test nor pituitary MRI provided unambiguous information about the source of ACTH hypersecretion whereas the use of selective blood sampling allowed to confirm the primary diagnosis of Cushing's disease in 4 patients and revise it in 2 others. In all the patients having the diagnosis established based on the results of selective blood sampling, it was confirmed after transsphenoidal adenomectomy. At the same time, the diagnosis of ACTH-ectopic syndrome was confirmed by an immunohistochemical method only in 1 of the 2 patients. Thus, the results of the present study indicate that selective blood sampling from the inferior petrosal sinuses is a valuable diagnostic tool which should be recommended for a wider application in endocrinological practice. However, this method failed to reveal lateralization of the tumours in all the examined patients.