Purpose: To show a rare case of Cushing’s disease and possible cause of failed transsphenoidal surgery.Method: We report on a 50-year-old woman suffering from ACTH-dependent Cushing’s syndrome. Endocrinological work-up including low-dose/high-dose dexamethasone test (Liddle-test) and CRH test were clearly compatible with pituitary origin. Although an MRI showed no pituitary tumor, CRH-stimulated petrosal sinus sampling revealed a significant central-peripheral gradient in ACTH concentrations, rendering Cushing’s disease very likely. The patient underwent transsphenoidal surgery with negative exploration of the pituitary gland. After intraoperative re-evaluation of the preoperative MRI, a “polyp” at the bottom of the sphenoid sinus was identified. The intraoperative microscopic aspect as well as instantaneous sections and cytology of a biopsy confirmed an adenoma, which was then removed. Histological analysis demonstrated an ACTH-producing pituitary adenoma adjacent to respiratory mucous membrane consisting of ciliated epithelium with submucous connective tissue. Postoperatively, ACTH concentrations were decreased and intermittent hydrocortisone substitution treatment was initiated. At the 3-month follow up, Cushing’s stigmata were found to be alleviated and the hydrocortisone dosage could be reduced.Conclusion: Ectopic pituitary adenoma tissue causing Cushing’s disease is extremely rare but a potential cause for surgical failure or re-evaluation.