HIV and hepatitis B virus (HBV) coinfection increases HIV and HBV replication, hepatitis flares, and risk of progression to chronic HBV infection, cirrhosis, and hepatocellular carcinoma. HIV and HBV coinfection decreases frequency of hepatitis Be antibody (anti-HBe) and hepatitis B surface antibody (anti-HBs) seroconversion, increases risk of antiretroviral therapy-related hepatotoxicity, and reduces efficacy of HBV therapy. All newly diagnosed HIV patients should be screened for hepatitis A, B, and C viruses and vaccinated if not immune to hepatitis A or B viruses. HBV serology often is atypical in coinfection. Diagnosis of HBV coinfection in HIV infection is made on the basis of hepatitis B surface antigen (HBsAg)-positive, hepatitis B core antibody (anti-HBc total)-positive, anti-HBs-positive status. Alanine aminotransferase levels in coinfected patients often are not reliable markers of liver inflammation. HBV infection should always be treated if coinfected patients are receiving antiretroviral therapy, since immune reconstitution under antiretroviral therapy poses risk for immune-associated liver damage in these patients. This article summarizes a presentation on HIV and HBV coinfection made by Marion G. Peters, MD, at an International AIDS Society-USA Continuing Medical Education course in San Francisco in May 2007.