A 42-year-old man presented with classical symptoms of Cushing's disease. Endocrinological examinations indicated the presence of a adrenocorticotropin-(ACTH) producing pituitary adenoma. Magnetic resonance imaging showed a cystic sellar lesion, which had features of a Rathke's cleft cyst, whereas the adenoma was not depicted. The ratio of blood ACTH concentration between cubital vein, left, and right cavernous sinus was 1: 1.4: 20. During transsphenoidal surgery, in addition to the Rathke's left cyst, a tiny adenoma was detected beneath the left pituitary lobe. Cavernous sinus sampling is a useful auxiliary in the localization of an ACTH-producing adenoma, as in this case.