We measured the prevalence of HBV surface antigen (HBsAg) among male injection drug users (IDUs) in Detention, Tehran, Iran. A cross-sectional survey included 499 male IDUs arrested by police during a predetermined police sweep in Tehran (February, 2006). A questionnaire was filled out for each individual. Blood specimens were collected for HBsAg testing. Prevalence of HBsAg was 5.8% (95% CI 3.6-7.9). The majority of chronic HBV infections, 69.2%, were among adults age 25 to 34 years. The high prevalence of HBsAg highlights the need for special efforts to increase vaccination among adult populations at risk for HBV infection in order to reducing continuing transmission and stave off future high burden of disease.