In the United States, an estimated 1.1 million persons were living with human immunodeficiency virus (HIV) infection in 2006, of whom an estimated 232,700 were undiagnosed and unaware they were HIV infected. Adolescents and young adults aged 13-24 years represented 4.4% of the total but disproportionately comprised an estimated 9.9% of the undiagnosed cases. Early diagnosis of HIV infection facilitates medical interventions and enables infected persons to reduce high-risk behavior and the likelihood of further HIV transmission. To determine the extent to which adolescents are being tested for HIV, data from the 2007 Youth Risk Behavior Survey (YRBS) (the most recent data available) were analyzed. The results indicated that nationwide, 12.9% of all high school students had ever been tested for HIV. The prevalence of HIV testing increased with increasing grade and decreased with increasing age at first sexual intercourse. Prevalence of HIV testing was higher among female students (14.8%) than male students (11.1%), higher among non-Hispanic black students (22.4%) than Hispanic (12.7%) and non-Hispanic white students (10.7%), was higher among students who had ever had sexual intercourse (22.3%) than those who had never had sexual intercourse (4.0%), and among students who had ever had sexual intercourse. To decrease the number of undiagnosed HIV infections among adolescents and promote HIV prevention, CDC recommends that health-care providers offer HIV screening as part of routine medical care. High schools can support those screening efforts by including information on obtaining HIV testing in their HIV curricula.