Introduction There have been noted that not only hepatitis B virus (HBV) vertical but horizontal infection elicits a risk of persistent HBV infection even in healthy adulthood. There is little information on incidence rate of HBV infection in Japan.

Methods Firstly, we estimated age-specific incidence rate of HBV infection among blood donors in Osaka in 1990s from (1) the number of HBsAg sero-converting donors (57 persons) among 185 554 repeat donors who donated in Osaka between 1992 and 1993, and (2) age-specific ratio for acute HB incident cases reported in Japan. Secondly, we modified the age-specific rates in 1990s to estimate the rates in 2000s by using age-specific coefficients which were provided under the assumption that the incidence rate is proportional to the prevalence in the same population. In the assumption, we obtained the coefficients by comparing age-specific HBsAg positive rates between first time blood donors in 1990s and those in 2000s. Finally, we calculated the annual number of HBV horizontal infection in Japan in 2005, using age-specific Japanese population in 2005. Sensitivity analysis was performed by changing parameters we assumed.

Results The estimated HBV horizontal infection rates among Japanese aged 15–19, 20–24, ---, 60–64 were 2.4, 11.5, 18.4, 16.5, 11.9, 8.4, 8.8, 7.3, 8.8, 3.9 per 100 000 P-Y, respectively in 2005. The estimated number of HBV horizontal infection in Japan was 8,409 in 2005.

Conclusion HBV horizontal infection rate was high in young adulthood in the Japanese population.