Cambodia has one of the highest rates of HIV infection in Southeast Asia. At the end of 2002, HIV infection rates among Cambodian sex workers ranged from 14.8% to 28.8%. Tenofovir DF is a nucleotide reverse transcriptase inhibitor (NRTI) that was licensed for the treatment of HIV-1 infection by the United States Food and Drug Administration (FDA) in October 2001. This randomized clinical trial will determine if a daily oral 300 mg dose of tenofovir DF is safe and effective in preventing HIV-1 infection. This is a collaborative study between the University of California, San Francisco, the University of New South Wales, and the Ministry of Health of Cambodia.