A significant proportion of human immunodeficiency virus (HIV) infected patients are coinfected with hepatitis B virus (HBV). Currently available treatments for chronic hepatitis B (interferon [IFN]-α and lamivudine [3TC]) have limited long-term utility because of side effects or of the development of resistance. Tenofovir disoproxil fumarate (TDF) is a nucleotide analog with excellent activity in vitro against HBV, which is also active against 3TC-resistant HBV variants. In this 24-week pilot study, the anti-HBV activity of TDF was prospectively evaluated in a cohort of 6 HIV coinfected subjects for whom 3TC and IFN therapy had previously failed. At baseline, all patients were taking 3TC or FTC and were hepatitis B surface antigen and hepatitis B e antigen positive; 4 had cirrhosis. Baseline HBV load was 7.95 log10 copies/mL. By Weeks 12 and 24, HBV load had decreased by 3.1 log10 copies/mL and 4.3 log10 copies/mL, respectively. There was a transient increase of transaminases after the initiation of treatment. No patient developed HBe antibodies. TDF is a very promising drug for the treatment of chronic hepatitis B in HIV-infected individuals.