PURPOSE: To describe the characteristic CT findings of inverted papilloma (IP) and aspergillosis involving sinonasal cavities and to differentiate between them on CT. MATERIALS AND METHODS: We retrospectively reviewed CT images in 22 cases of pathologically confirmed IP and 16 of aspergillosis; these were classified as one of four types, according to location. We also analyzed infundibular widening, displacement of the maxillary medial wall, the presence of air in the maxillary ostium, elevation of ethmoidom axillary plate (EMP), calcification within a lesion, reactive hyperostosis and CT density. RESULTS: Four cases were type I, which was seen only in aspergillosis (4/16, 25%); five were type IV, and all were IP (5/22, 22.7%), with a characteristic growing vector. Type II was seen in nine cases of IP and 11 of aspergillosis, and type III in eight cases of IP and one of aspergillosis. In types II and III (17 of 22 cases of IP; 12 of 16 of aspergillosis), five of 22 cases of IP (22.7%) and nine of 16 of aspergillosis (56.3%) showed infundibular widening, which was more severe in aspergillosis. Maxillary medial wall displacement was seen in five of 22 cases (22.7%) of IP, four of which showed lateral displacement and in five of 16 cases (31.3%) of aspergillosis, all of which showed medial displacement. Air in the maxillary ostium was seen in seven of 22 cases of IP (31.8%). Elevation of EMP was seen in two cases of IP (9.1%) and three of aspergillosis (18.8%), which showed oblique elevation. Calcification was detected in nine cases of aspergillosis (56.3%) but in only one of IP. Reactive hyperostosis was seen in 13 cases of aspergillosis (81.3%) and two of IP (9.1%). CONCLUSION: Although it is hard to differentiate between IP and aspergillosis involving sinonasal cavities, the findings of calcification, infundibular widening, air in the maxillary ostium,displacement of the maxillary medial wall, an elevated EMP pattern and reactive hyperostosis combined with duration of symptom may be helpful.