Immunoreactivity of proliferating cell nuclear antigen (PCNA), Ki67 and p53 in inflammatory nasal polyp and inverted papilloma tissues was investigated. Immunohistochemistry was performed using a standard avidin-biotin-peroxidase method, and the immunoreactivity of PCNA, Ki67 and p53 was quantified by counting immunostained nuclei in at least 1000 epithelial cells. The mean labelling index (percentage of immunostained cells) for PCNA was 40.68 in the inverted papilloma group and 14.73 in the nasal polyp group, and for Ki67 was 15.43 in the inverted papilloma group and 2.64 in the nasal polyp group. Both of these differences between the inverted papilloma and nasal polyp groups were significant. Immunoreactivity for p53 was detected in five (35.7%) inverted papilloma patients and two (9.5%) nasal polyp patients. The increase in epithelial cell proliferation seemed to be greater in inverted papillomas than in inflammatory nasal polyps. Increased epithelial cell proliferation may be involved in the development of inverted papillomas.