Ectopic adrenocorticotrophic hormone (ACTH) secretion (EAS) is a rare cause of ACTH-dependent Cushing's syndrome (CS), most often caused by a thoracic neuroendocrine tumor (NET). Large-cell neuroendocrine carcinomas (LCNEC) with EAS are rare and usually present a more severe ACTH secretion and hypercortisolism. We report a 44-year-old non-smoker man, who presented clinical and biochemical evidence of ACTH-dependent CS. Desmopressin 10 μg i.v. produced a 157% increase in ACTH and a 25% increase in cortisol from baseline; there was no stimulation of ACTH or cortisol during the corticotropin-releasing hormone (CRH) test and no suppression with high dose dexamethasone. Pituitary MRI identified a 5 mm lesion, but inferior petrosal venous sinus sampling under desmopressin did not identify a central ACTH source. Thorax and abdominal imaging identified a left lung micronodule. Surgery confirmed a lung LCNEC with strongly positive ACTH immunohistochemistry (IHC) in the primary and lymph node metastasis. The patient was in CS remission after surgery and adjuvant chemotherapy but developed a recurrence 9.5 years later, with LCNEC pulmonary left hilar metastases, ectopic CS, and positive ACTH IHC. This is the first report of LCNEC, with morphologic feature of carcinoid tumor of the lung with ectopic ACTH stimulated by desmopressin. Long delay prior to metastatic recurrence indicates relatively indolent NET. This case report indicates that response to desmopressin, which usually occurs in Cushing's disease or benign NETs, can occur in malignant LCNEC.