A 62-year-old man had a malignant prolactin-secreting pituitary adenoma. The tumor was initially resected through a transfrontal approach. Recurrence six years later was manifested by an occipital metastasis, which was excised. Progressive clinical deterioration ensued despite irradiation and chemotherapy of the intrasellar and occipital recurrence, and the patient died. Pathologic examination of tumor tissue obtained from the occipital metastasis showed increased cellular pleomorphism and mitotic rate in comparison with the original surgical specimen. The only true indication of malignant transformation, however, was the invasion of tumor capsule by adenoma cells, observed in the autopsy specimen. These findings support the contention that the diagnosis of a malignant pituitary adenoma rests ultimately on the demonstration of local invasive growth and/or distant metastasis. Since prolactinomas are usually non-aggressive, malignant degeneration of adenomas is decidedly rare. Its occurrence underlines the need for aggressive follow-up and treatment.