Despite diagnostic advances, it remains difficult to identify intrasellar and ectopic parasellar adrenocorticotropic hormone (ACTH)-secreting microadenomas. The authors present the case of a 61-year-old woman with Cushing disease in whom a significant central-to-peripheral and lateralized right-sided ACTH gradient was demonstrated on inferior petrosal sinus sampling; no discernible abnormality was seen on magnetic resonance imaging. She underwent transnasal transsphenoidal surgery. No tumor was found on sellar exploration and a total hypophysectomy was performed, yet her hypercortisolemia persisted. The patient died of cardiac events 17 days postsurgery. Autopsy revealed an isolated, right-sided, intracavernous ACTH-secreting adenoma with no intrasellar communication. This case represents the first failed transsphenoidal surgery for Cushing disease in which there is postmortem confirmation of a suspected intracavernous sinus lesion. It supports the hypothesis that Cushing disease associated with nondiagnostic imaging studies, a strong ACTH gradient on venography, and negative findings on sellar exploration may be caused by an ectopic intracavernous ACTH-secreting adenoma. There are no premortem means of confirming the presence of such lesions, but these tumors could underlie similar cases of failed surgery. Radiation therapy targeting the sella turcica and both cavernous sinuses, possibly supplemented with medical treatment, is suggested for similar patients in whom transsphenoidal hypophysectomy has failed. Adrenalectomy may also be appropriate if a rapid reduction in ACTH is necessary.