There are many potential reasons for increased liver-related mortality in HIV-hepatitis B virus (HBV) coinfection compared with either infection alone. HIV infects multiple cells in the liver and might potentially alter the life cycle of HBV, although evidence to date is limited. Unique mutations in HBV have been defined in HIV-HBV-coinfected individuals and might directly alter pathogenesis. In addition, an impaired HBV-specific T-cell immune response is likely to be important. The roles of microbial translocation, immune activation and increased hepatic stellate cell activation will be important areas for future study.