We report here a case of ectopic pituitary adenoma with malignant transformation after repeated relapses. First, an ectopic pituitary adenoma producing follicle-stimulating hormone was found in the nasal cavity extending to the frontal cranial fossa. Despite repeated surgical resections of the tumor, it recurred three times in 2 years. The tumor gradually showed cellular atypia, mitosis, and necrosis. Immunohistochemical analyses revealed that the expressions of proliferating cell nuclear antigen and MIB-1 increased progressively. Moreover, the expression of p53 was detected at the second recurrence. Finally, at the third recurrence the tumor showed dissemination to the subarachnoid space and multiple metastases in the brain. The patient died of the tumor 10 months after the last resection. These findings indicate that the ectopic pituitary adenoma became malignant. To our knowledge, this is the first report on malignant transformation of ectopic pituitary adenoma. It is important to know that ectopic pituitary adenomas show malignant transformation and that the above parameters (proliferating cell nuclear antigen, MIB-1, and p53) may be useful indicators of the malignant potential of both ectopic and sellar pituitary tumors.