A program of mass hepatitis A+B vaccination in preadolescents in schools was begun in the Catalonia in the last quarter of 1998. This study investigated the impact of the program by comparing the incidence of hepatitis A in vaccinated and unvaccinated cohort. The greatest reduction of the incidence rate of hepatitis A was observed in the 10-14 years age group, from 10.3 per 100000 persons-year in the period 1996-1998 to 1.8 per 100000 persons-year in the period 1999-2001. The global incidence decreased from 6.2 to 2.6 per 100000 persons-year. After analysis of cases occurring in the vaccinated and non vaccinated cohort, the effectiveness of the vaccination program was estimated at 97.0% (95% CI: 78.6-99.6).