Objectives/Hypothesis: Sinonasal inverted papilloma is a rare but locally aggressive benign tumor. Inverted papilloma tends to recur after surgical resection and is occasionally associated with squamous cell carcinoma. Radical en bloc resection by lateral rhinotomy and medial maxillectomy has been recommended for initial management; however, endoscopic sinus surgery is effective in selected cases. To determine adequate surgical approaches, a staging system for inverted papilloma based on extent and location of the tumor has been advocated. The study investigated whether preoperative assessment by magnetic resonance imaging (MRI) accurately predicts the extent of inverted papilloma. Study Design: Magnetic resonance imaging was retrospectively reviewed in 21 cases of inverted papilloma without knowledge of the surgical and pathological findings. Methods: Patients were categorized into stages based on MRI findings, according to the staging system proposed by Krouse. The involvement of inverted papilloma in each sinus was also assessed. Results: Stages graded by MRI were coincident with postoperative staging verified by surgical and pathological findings in 18 of the 21 cases (86%), which included 1 case of stage T1, 3 cases of stage T2, and 14 cases of stage T3. Two cases of stage T2 were judged as T3, and one case of T3 was judged as T2 by MRI. Positive predictive value of MRI diagnosis for tumor involvement was 68% to 89% in each sinus, and negative predictive value was more than 93%. Conclusion: In most cases, MRI assessment of inverted papilloma can accurately predict the extent of tumor involvement. Preoperative staging of inverted papilloma by MRI may be useful for planning an appropriate surgical approach.