Ectopic ACTH secretion accounts for less that 10% of all causes of endogenous Cushing's syndrome. Carcinoids are rare thymic tumors, and when associated with ACTH hypersecretion display local or distant aggressive behavior. A 32-year-old woman was admitted to the Endocrinology Unit for obesity, moon face, facial hirsutism, hyperpigmentation, and secondary amenorrhea. Laboratory test confirmed the hypercortisolism and excess ACTH, while dexamethasone suppressive test was negative. Thorax computed tomography (CT) showed an antero-superior mediastinal tumor invading the pericardium and left mediastinal pleura. A complete resection through median sternotomy of the tumor, pericardium and left mediastinal pleura was performed. After a one-year symptom-free period, hypercortisolism recurred, confirmed by laboratory findings. Although no signs of local recurrence were seen on thorax CT, left internal mammary lymph nodes involvement and vertebral body metastases at C7 and LI were found. Refractory electrolyte disturbances could not be corrected resulting in severe cardiac arrhythmia and death from cardiac arrest. The reported case draws attention on the aggressiveness of ACTH-secralso due to the refractory electrolyte disturbances with fatal outcome.