Nine hundred and sixty HIV uninfected female sex workers in Phnom Penh will be enrolled in the trial. Participants will be randomized to receive either 300 mg of tenofovir DF or placebo daily for 12 months. Participants will be evaluated for rates of HIV infection, adherence to the drug regimen, and changes in risk behaviors. All participants will be monitored throughout the trial for side effects and toxicity. Participants will be involved in the study for 14 months.