Priority health risk behaviors that contribute to the leading causes of mortality, morbidity, and social problems among youth and adults often are established during youth, extend into adulthood, and are interrelated. This report summarizes results from the Youth Risk Behavior Surveillance System (YRBSS). The 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, dietary behaviors, and physical activity. The YRBSS includes a national, school-based survey conducted by the Centers for Disease Control and Prevention and State and local school-based surveys conducted by State and local education agencies. This report summarizes results from the national survey, 24 State surveys, and nine local surveys conducted among high school students during February through May 1993. Results indicate that in the United States, 72 percent of all deaths among school-age youth and young adults are from four causes: motor vehicle crashes, other unintentional injuries, homicide, and suicide. Many high school students practice behaviors that may increase their likelihood of death from these four causes. Substantial morbidity and social problems among adolescents also result from unintended pregnancies and sexually transmitted diseases, including human immunodeficiency virus infection. YRBSS results indicate that in 1993, 53 percent of high school students had had sexual intercourse. Among adults, 67 percent of all deaths are from three causes: heart disease, cancer, and stroke. Many high school students also practice behaviors that may increase the risk for these health problems. Methodology used in the study is explained. Appendixes include a listing of state and local YRBSS coordinators and state and territorial epidemiologists and laboratory directors. Tables