Inverted papilloma (IP) is a primarily benign epithelial neoplasm with a propensity to transform to squamous cell carcinoma (SCC). Neither the etiology of IP nor the factors responsible for malignant transformation are fully known to date. A considerable number of histopathological grading systems have been suggested. It was the aim of this study to find histological parameters in IP that allow a prognosis concerning the occurrence of malignant transformation. In a group of 93 cases of IP, the patients' records, histological specimens and a questionnaire were evaluated. Thirteen patients had suffered a recurrence of a previously treated IP. SCC in the IP had been diagnosed in 12 of the cases. The histological specimen showed increased counts of mitosis and dyscariosis in the IP. All other histological parameters did not show a significant difference between the groups. History and symptoms proved to be nonspecific as well. Patients in the group with SCC were significantly older at the first diagnosis of IP than patients without carcinoma. This group also included a higher portion of male patients. Patients fulfilling the named histological criteria combined with male gender and higher age are a high-risk group in the necessary long-term follow-up.