Priority health-risk behaviors, which contribute to the leading causes of mortality and morbidity among youth and adults, often are established during youth, extend into adulthood, and are interrelated. The Youth Risk Behavior Surveillance System (YRBSS) monitors six categories of priority health-risk behaviors among youth and young adults--behaviors that contribute to unintentional and intentional injuries; tobacco use; alcohol and other drug use; sexual behaviors that contribute to unintended pregnancy and sexually transmitted diseases (STDs) (including human immunodeficiency virus [HIV] infection); unhealthy dietary behaviors; and physical inactivity. The YRBSS includes a national school-based survey conducted by the Centers for Disease Control and Prevention as well as state, territorial, and local school-based surveys conducted by education and health agencies. This report summarizes results from the national survey, 33 state surveys, 3 territorial surveys, and 17 local surveys conducted among high school students from February through May 1997. In the United States, 73% of all deaths among youth and young adults 10-24 years of age result from only four causes: motor vehicle crashes, other unintentional injuries, homicide, and suicide. Results from the national 1997 YRBSS demonstrate that many high school students engage in behaviors that increase their likelihood of death from these four causes--19.3% had rarely or never worn a seat belt; during the 30 days preceding the survey, 36.6% had ridden with a driver who had been drinking alcohol; 18.3% had carried a weapon during the 30 days preceding the survey; 50.8% had drunk alcohol during the 30 days preceding the survey; 26.2% had used marijuana during the 30 days preceding the survey; and 7.7% had attempted suicide during the 12 months preceding the survey. Substantial morbidity among school-age youth, young adults, and their children also result from unintended pregnancies and STDs, including HIV infection. YRBSS results indicate that in 1997, 48.4% of high school students had ever had sexual intercourse; 43.2% of sexually active students had not used a condom at last sexual intercourse; and 2.1% had ever injected an illegal drug. Of all deaths and substantial morbidity among adults greater than or equal to 25 years of age, 67% result from two causes--cardiovascular disease and cancer. Most of the risk behaviors associated with these causes of death are initiated during adolescence. In 1997, 36.4% of high school students had smoked cigarettes during the 30 days preceding the survey; 70.7% had not eaten five or more servings of fruits and vegetables during the day preceding the survey; and 72.6% had not attended physical education class daily. These YRBSS data are already being used by health and education officials to improve national, state, and local policies and programs to reduce risks associated with the leading causes of morbidity and mortality. YRBSS data also are being used to measure progress toward achieving 21 national health objectives and one of the eight National Education Goals.