Unprotected sexual intercourse places young persons at risk for human immunodeficiency virus (HIV) infection, other sexually transmitted diseases (STDs), and unintended pregnancy. Responsible sexual behavior among adolescents is one of the 10 leading health indicators of the national health objectives for 2010 (objective 25.11). To examine changes in sexual risk behavior that occurred among high school students in the United States during 1991-2001, CDC analyzed data from six national Youth Risk Behavior surveys (YRBS). This report summarizes the results of the analysis, which indicate that, during 1991-2001, the percentage of U.S. high school students who ever had sexual intercourse and the percentage who had multiple sex partners decreased. Among students who are currently sexually active, the prevalence of condom use increased, although it has leveled off since 1999. However, the percentage of these students who used alcohol or drugs before last sexual intercourse increased. Despite decreases in some sexual risk behaviors, efforts to prevent sexual risk behaviors will need to be intensified to meet the national health objective for responsible sexual behavior.