ALTHOUGH the inverted papilloma of the nasal cavity and paranasal sinuses is a neoplasm that is histologically benign by the usual pathologic definition, experience with a number of patients over a period of time has proved it deserves a classification of "clinically malignant." Even though this is not a rare condition and has received detailed study, a wide recognition of the frequency of this tumor and its destructive clinical course has not been evident. Bone erosion and invasion of adjacent soft tissue frequently occur because persistence or recurrence has followed an inadequate operation for a tumor too generally accepted as an inflammatory or "fleshy" polyp with metaplasia. Where repeated removal of recurrent polypoid tissue permits a review of the previous histological material, the characteristic features of inverted papilloma have been found to be present in the earlier specimens. It is the rapid recurrence of the nasal tumors and the accompanying