A 49-yr-old woman with a large pituitary tumor leading to visual loss and galactorrhea- amenorrhea was submitted to transcranial pituitary surgery, when a clinically nonfunctioning pituitary adenoma was partially removed. Histopathology and immunohistochemistry confirmed the diagnosis of "non-secreting atypical adenoma." At that time, serum and urinary free cortisol were normal, with low T4 levels and hyperprolactinemia. The patient was discharged on thyroxine and bromocriptine and treated with conventional radiotherapy. Two years later, she presented high free urinary cortisol levels and a positive ACTH response to desmopressin testing on dexametasone 2 mg overnight. A pituitary biopsy confirmed aggressive growth as well as positive immunoreactivity for ACTH, p53, Ki-67, and c-erb-B2. The patient was then treated with radiosurgery on ketoconazole therapy. The overall clinical, laboratory, and pathological data suggest a transition from a clinically nonfunctioning to a hypersecreting ACTH-producing tumor. Putative mechanisms of tumor transformation and the possibility of a silent corticotropinoma evolving into clinical Cushing s syndrome are discussed.