Sinonasal inverted papilloma (SNIP) is a benign epithelial tumor with distinctive histopathological features. However, the role of inflammation in SNIP remains poorly characterized.This study aimed to compare the histopathological patterns and inflammatory characteristics of SNIP with those of chronic rhinosinusitis with nasal polyps (CRSwNPs) or normal ethmoid sinus mucosa.Fifty-eight tissue biopsies were prospectively collected from 38 patients with SNIPs, 12 CRSwNPs, and 8 normal ethmoid sinus mucosae. SNIP was histopathologically divided into four grades based on the extent of epithelial remodeling. The immunohistochemical characteristics of epithelial remodeling (p63, CK5) and infiltration of inflammatory cells (eg, eosinophils, neutrophils, and macrophages) and cytokines (eg, interleukin-1β, interleukin-6, and tumor necrosis factor-α) were analyzed.Among the 38 SNIPs, 21.1%, 36.8%, 23.7%, and 18.4% were grades I, II, III, and IV, respectively. The expression levels of p63 and CK5 were significantly higher in SNIP than in the other two groups (both, p < 0.05). Neutrophil and macrophage infiltration was more pronounced in SNIP and with differences among the four grades. The expression levels of inflammatory cytokines were significantly higher in the SNIP group than in the CRSwNP group. A positive correlation between the expression levels of p63 and inflammatory cytokines was observed in both SNIPs and CRSwNPs.Excessive epithelial remodeling is an important histological feature of SNIP; it is accompanied by sinonasal mucosal inflammation.