Cushing's syndrome is the common clinical presentation of three unique disorders that give rise to hypercortisolism. In most cases neoplasms underly each of these disorders. Clinical features are highly variable and not accounted for by cortisol alone; indeed, the multihormonal basis for much of the clinical syndrome remains uncertain. Demonstration of sustained, excessive cortisol production is essential and depends on a pattern of repeated measurements and several different procedures. Plasma adrenocorticotropin, although not helpful in establishing the diagnosis, has proved valuable in differentiating the three major entities that cause hypercortisolism. The renewed significance of pituitary microadenomas and their improved detection by sella tomography has accompanied recent, impressive advances in transsphenoidal microsurgery. This may become the preferred treatment for pituitary Cushing's syndrome in the adult, particularly where the appropriate equipment and skills are available; for children, external pituitary irradiation seems to offer safe and effective therapy.