One of the three primary goals of the National HIV/AIDS Strategy for the United States is to reduce the number of persons who become infected with human immunodeficiency virus (HIV). In 2009, persons aged 15-29 years comprised 21% of the U.S. population but accounted for 39% of all new HIV infections. Sexual intercourse, sexual intercourse with multiple partners, sexual intercourse without using a condom, and injection drug use are behaviors that increase risk for HIV infection. To describe trends in the prevalence of HIV-related risk behaviors among high school students, CDC analyzed data from the biennial national Youth Risk Behavior Survey (YRBS) for the period 1991-2011. The results of that analysis indicated that, although the percentage of students overall who had ever had sexual intercourse decreased significantly from 54.1% in 1991 to 47.4% in 2011, the prevalence of ever having had sexual intercourse did not change significantly after reaching 45.6% in 2001. Similarly, although the percentage of students who had four or more sex partners decreased significantly from 18.7% in 1991 to 15.3% in 2011, the prevalence of having four or more sex partners did not change significantly after reaching 14.2% in 2001. Condom use at most recent sexual intercourse among students currently having sexual intercourse increased from 46.2% in 1991 to 60.2% in 2011. However, the prevalence of condom use did not change significantly beginning in 2003 (63.0%). The prevalence of injection drug use among students overall did not change significantly from 1995 (2.1%) to 2011 (2.3%). The results suggest that progress in reducing some HIV-related risk behaviors among high school students overall and in certain populations did not change significantly in the past decade. To reduce the number of young persons who become infected with HIV, renewed educational efforts and other risk reduction interventions are warranted.