Adolescents and young adults are at high risk for human papillomavirus (HPV) and hepatitis B virus (HBV) infections, which are preventable by currently available, safe and effective, prophylactic vaccines. However, development of a combined immunization strategy may lead to better compliance for these vaccines, thereby contributing to the overall goal of protection against these diseases. This study assessed the safety and immunogenicity of co-administered quadrivalent HPV-6/11/16/18 L1 VLP and HBV vaccines in women (n=1877) aged 16-23 years. Co-administration of HPV and HBV vaccines induced robust anti-HPV-6, HPV-11, HPV-16, HPV-18 geometric mean titers (GMTs) and > or =99% seroconversion rates (Month 7) that were both non-inferior (p<0.001) to those induced by HPV vaccine alone. High Month 7 anti-HBs GMTs were also observed following concomitant vaccination. These GMTs were lower compared to those induced by the HBV vaccine itself; however, >96% of subjects achieved an anti-HBs seroprotection level of > or =10 mIU/mL that was non-inferior (p<0.001) to that of HBV vaccine alone. Overall, co-administered and individual vaccines were generally well-tolerated and did not interfere with the immune response of either vaccine (ClinicalTrials.gov number, NCT00092521).