OBJECTIVES: To estimate the potential public health impact of a 9-valent HPV (human papillomavirus) vaccine (HPV9) in Japan in preventing HPV-related diseases including cervical intraepithelial neoplasia grades 1 (CIN 1), grades 2 and 3 (CIN 2/3), and cervical cancer. METHODS: A mathematical model of the transmission dynamics of HPV infection and disease for the HPV9 vaccine was calibrated to Japanese epidemiological data on cervical cancer.  Based on global estimates, we attributed 70% of cervical cancer to HPV 16 and 18 for the quadrivalent vaccine (types 6/11/16/18: HPV4), and 20% to the five additional types in HPV9 (31, 33, 45, 52, and 58). Other inputs were from public data sources and published literature. Vaccine efficacy against the 5 additional HPV types was taken from phase III trial results for the HPV9 vaccine. We assessed the incremental public health impact of HPV9 over HPV4 based on vaccinating 80% of females by age 12. RESULTS: We projected that HPV9 vaccination of females could reduce the incidence of cervical cancer by 46% over 100 years, relative to 36% for HPV4.  HPV9 vaccination relative to HPV4 vaccination could prevent an additional 305,000 cases of CIN1, 636,000 cases of CIN2/3, and 78,000 cases of cervical cancer in the Japanese population, cumulative over 100 years. CONCLUSIONS: Protecting the Japanese population against HPV infection with an HPV9 vaccination program relative to an HPV4 vaccination program can have significant public health benefits in addition to the benefits provided by HPV4 vaccination.