Objectives: This study aimed to determine the prevalence of hepatitis B virus (HBV) coinfection and HBV seropositivity in perinatally HIV-infected adolescents. A secondary objective was to describe the clinical characteristics of adolescents with chronic HBV/HIV coinfection. Materials And Methods: Multicenter cross-sectional study of perinatally HIV-infected adolescents aged 12-25 years. HBV surface antigen, surface antibody (anti-HBs) and core antibody (anti-HBc) were measured. Coinfection was defined as having persistently positive HBV surface antigen. Seroprotective antibody from immunization was defined as having anti-HBs â‰1 10 mIU/mL with negative anti-HBc. HBV DNA quantitation and rtM204V/I mutation analysis (lamivudine resistance-associated mutation) were performed in adolescents with chronic HBV infection. Results: From November 2010 to March 2011, 521 patients were enrolled. Mean (SD) of CD4 lymphocyte count was 685 (324) cells/μL. The prevalence of HBV/HIV coinfection was 3.3% (95% confidence interval: 1.9-5.2%). Protective antibody against HBV was found in 18% of population, and this was significantly higher among adolescents who received than those who did not receive HBV revaccination after receiving antiretroviral therapy (93% versus 6%, P < 0.01). Among adolescents with chronic HBV infection, 88% have received lamivudine; however, 69% have HBV DNA > 10 copies/mL and 75% had the rtM204V/I mutation. Conclusions: The prevalence of HBV coinfection in HIV-infected Thai adolescents was 3.3%. Most HIV-infected adolescents had no HBV protective antibody; therefore, revaccination with HBV vaccine is encouraged. The high prevalence of HBV-lamivudine resistance underscores the importance of HBV screening prior to antiretroviral therapy initiation to guide the selection of optimal regimen for coinfected children. © 2012 by Lippincott Williams and Wilkins.