BACKGROUND The goal of this study was to determine the prevalence and risk factors for horizontal transmission of hepatitis B infection due to intravenous drug abuse (IVDA), tattooing, blood transfusion, and combinations of these risk factors. METHODS All people detained in 19 prisons were invited to participate. Subjects underwent a physical examination and completed a questionnaire. Blood samples were collected and screened for hepatitis B surface antigen and Venereal Disease Research Laboratory (VDRL) reactivity. RESULTS A total of 16 204 prisoners were invited to participate. The response rate was 92.6% and the mean age was 29.73 years. The overall prevalence of HBsAg positivity was 21.7%. Among 3333 subjects with a history of blood transfusion, the carrier rate was 23.19%. Among the 3071 subjects with a history of IVDA and the 6908 subjects with tattoos, the carrier rate was 26.4% and 29.3%, respectively. IVDA appeared to be the strongest risk factor among the 3, with an adjusted odds ratio (AOR) of 1.54 (95% confidence interval, 1.27-1.86), followed by tattooing (1.40, 1.23-1.55), and blood transfusion (1.27, 1.05-1.61). When blood transfusion was combined with either of the other 2 risk factors, the risk increased multiplicatively, and the combination of the 3 factors increased the AOR to 2.76 (2.20-3.47). The prevalence of a positive VDRL test result was 1.01%. CONCLUSIONS The prevalence of HBV antigenemia in prisoners was high and was associated with BT, IVDA, and tattooing. A national vaccination program against HBV should be considered for prisoners and other people with these risk factors.