An estimated 11 000 new human immunodeficiency virus (HIV) infections occur worldwide per day and approximately 4 million individuals are infected with HIV per year.1 Although behavior change has likely led to substantial reductions in HIV incidence in some populations and risk-reduction counseling will likely remain the cornerstone of HIV prevention programs, new HIV prevention strategies are urgently needed to further reduce incident infections. Preexposure chemoprophylaxis (PrEP) has emerged as a promising new biomedical strategy for preventing HIV infection,2 and clinical trials are planned or under way3 to evaluate the safety and efficacy of this approach. Because many antiretroviral drugs are licensed in the United States, PrEP could become available for use as a prevention tool more quickly than other experimental prevention strategies, such as an HIV vaccine.