The researchers recruited patients with stable, low levels of detectable virus (HIV-1 RNA levels between 400 and 10 000 copies/mL). The researchers randomly assigned these patients to receive placebo or tenofovir DF (300 mg once daily). Neither the researchers nor the patients knew who got which treatment. During the study, all patients continued their other antiretroviral drug therapy. The researchers monitored viral loads, measures of immune function (CD4 cell counts), and side effects for 6 months. Researchers also did special genotyping tests to help assess resistance to drugs in about half of the patients.