OBJECTIVE: To determine the duration of protection from hepatitis B vaccine given in infancy and early childhood. DESIGN: Cross sectional serological study of hepatitis B virus infection in children of various ages 14 years after the start of a trial of vaccination regimens. SETTING: Two villages in the Gambia. PARTICIPANTS: Children and adolescents given hepatitis B vaccine in infancy or early childhood: 232 were aged 1-5 years, 225 aged 5-9 years, 220 aged 10-14 years, and 175 aged 15-19 years. MAIN OUTCOME MEASURES: Vaccine efficacy against infection and against chronic infection in the different age groups. RESULTS: Vaccine efficacy against chronic carriage of hepatitis B virus was 94% (95% confidence interval 89% to 97%), which did not vary significantly between the age groups. Efficacy against infection was 80% (76% to 84%). This was significantly lower in the oldest age group (65%, 56 to 73). Of the uninfected participants in this age group, 36% had no detectable hepatitis B virus surface antibody. Time since vaccination and a low peak antibody response were the most powerful risk factors for breakthrough infection (P<0.001 in each case). Low peak antibody response was also a risk factor for chronic carriage (odds ratio 95, 19 to 466). CONCLUSIONS: Children vaccinated in infancy are at increased risk of hepatitis B virus infection in the late teens. The risk of chronic carriage after sexual exposure needs further assessment to determine if booster vaccines are necessary.