Each year, approximately three million cases of sexually transmitted diseases (STDs) occur among teenagers, and approximately one million become pregnant. Human immunodeficiency virus (HIV) infection is the sixth leading cause of death among persons aged 15-24 years in the United States. Unprotected sexual intercourse and multiple sex partners place young persons at risk for HIV infection, other STDs, and pregnancy. To determine trends in sexual risk behaviors among high school students, CDC analyzed data from the Youth Risk Behavior Survey (YRBS) for the years 1991, 1993, 1995, and 1997. This report summarizes the results of this analysis, which indicate that, from 1991 to 1997, the percentage of U.S. high school students who had ever had sexual intercourse decreased, and the prevalence of condom use among currently sexually active students increased.