Leptomeningeal dissemination of pituitary adenoma is a very rare occurrence. The present report describes the case of a 28 year old man with a nonfunctioning pituitary adenoma which was operated on and irradiated. Eight years later, the patient developed Cushing's syndrome and multiple leptomeningeal masses were revealed by brain CT and MNR. The diagnosis was ACTH-cell adenoma, without malignant histological signs. The growth fraction of the tumor, detected by means of the immunohistochemical demonstration of proliferating cell nuclear antigen (PCNA), was 5.45% of cells; this figure is higher than that reported for non-recurrent pituitary adenomas. From a review of the reported cases, the possibility of predicting late malignant behaviour is discussed. The microscopic aspect has no prognostic value, since metastasizing cases are not overtly malignant in a histological or cytological sense. The application of methods aimed at detecting the growth fraction of the tumor may prove useful in the early identification of aggressive pituitary tumors.