Preoperative radiological assessment of sinonasal inverted papilloma (SNIP) is important in the planning of surgical treatment. This study investigates the roles and limitations of preoperative plain computed tomography (CT) scan in the preoperative assessment of SNIP. Plain CT scans from 30 patients with SNIP were reviewed retrospectively by a radiologist who had no prior knowledge of the final surgical findings. Disease at each sinus was judged by the CT findings of opacity and additional signs. The overall disease was staged according to the staging system proposed by Krouse. All of the findings were compared with the final disease extent and staging confirmed by intraoperative and histological findings. Using opacity with additional signs for diagnosis, the range of accuracy of CT diagnosis for each sinus involvement was 83-97%. Staging by plain CT was concordant with postoperative staging in 80% of patients. Among the additional signs, focal hyperostosis or "bony strut" had the highest positive predictive value (100%) of tumor origin. Focal hyperostosis or bony strut is the most important CT sign predicting the origin of tumor. Although using multiple CT diagnostic signs provides a reasonable assessment of tumor origin and extent, accurate tumor mapping was still impossible because of inadequate differentiation of tumor from inflammatory pathologies. This drawback may be overcome by a complementary MRI scan. Since preoperative CT staging was inaccurate in 20% of cases, surgical planning should be flexible to provide for the need of the intraoperative findings.