Patients are randomized 2:1 to add tenofovir DF once daily or placebo to their existing antiretroviral regimen in a blinded manner. Patients are stratified according to HIV-1 RNA level, CD4 cell count, and number of antiretroviral drugs taken prior to study entry. Patients and physicians are strongly discouraged from making changes in their antiviral therapies for at least 24 weeks post-randomization. After Week 24, changes in background antiretroviral therapy are permitted. At 24 weeks post-randomization, patients randomized to receive placebo are crossed over to receive open-label tenofovir DF once daily for the remainder of the 48-week study. While on study drug, patients are monitored for safety using periodic physical examinations and serial laboratory tests. Additionally, changes in plasma HIV RNA levels and CD4 cell counts are monitored to assess antiviral efficacy. At the end of the 48-week study period, patients are rolled over into extended dosing until tenofovir DF is commercially available or Gilead Sciences terminates the study.