Inverted papillomas of the paranasal sinuses have been characterized by their unusually benign histologic features, their ability to grow rapidly with bony destruction, and their tendency to recur if not adequately treated. The association of inverted papilloma with squamous cell carcinoma is well described, but malignant transformation is relatively rare. We report a case of a multiply recurrent inverted papilloma that spread to the middle ear and mastoid and eventually underwent malignant transformation with skull base invasion. Pathologic examination demonstrated many of the characteristics associated with malignant transformation. In addition, progesterone receptors were demonstrated that may have stimulated this tumor during the patient's pregnancy. To our knowledge, no similar cases have been reported in the literature. © 1995, American Medical Association. All rights reserved.