Problem: Priority health-risk behaviors, which contribute to the leading causes of morbidity and mortality among youth and adults, often are established during childhood and adolescence, extend into adulthood, are interrelated, and are preventable. Reporting Period Covered: October 2004-January 2006. Description of the System: The Youth Risk Behavior Surveillance System (YRBSS) monitors six categories of priority health-risk behaviors among youth and young adults, including behaviors that contribute to unintentional injuries and violence; tobacco use; alcohol and other drug use; sexual behaviors that contribute to unintended pregnancy and sexually transmitted diseases (STDs), including human immunodeficiency virus (HIV) infections; unhealthy dietary behaviors; and physical inactivity. In addition, the YRBSS monitors general health status and the prevalence of overweight and asthma. YRBSS includes a national school-based survey conducted by CDC and state and local school-based surveys conducted by state and local education and health agencies. This report summarizes results from the national survey, 40 state surveys, and 21 local surveys conducted among students in grades 9-12 during October 2004-January 2006. Results: In the United States, 71% of all deaths among persons aged 10-24 years result from four causes: motor-vehicle crashes, other unintentional injuries, homicide, and suicide. Results from the 2005 national Youth Risk Behavior Survey (YRBS) indicated that, during the 30 days preceding the survey, many high school students engaged in behaviors that increased their likelihood of death from these four causes: 9.9% had driven a car or other vehicle when they had been drinking alcohol; 18.5% had carried a weapon; 43.3% had drunk alcohol; and 20.2% had used marijuana. In addition, during the 12 months preceding the survey, 35.9% of high school students had been in a physical fight and 8.4% had attempted suicide. Substantial morbidity and social problems among youth also result from unintended pregnancies and STDs, including HIV infection. During 2005, a total of 46.8% of high school students had ever had sexual intercourse; 37.2% of sexually active high school students had not used a condom at last sexual intercourse; and 2.1% had ever injected an illegal drug. Among adults aged greater than 25 years, 61% of all deaths result from two causes: cardiovascular disease and cancer. Results from the 2005 national YRBS indicated that risk behaviors associated with these two causes of death were initiated during adolescence. During 2005, a total of 23.0% of high school students had smoked cigarettes during the 30 days preceding the survey; 79.9% had not eaten greater than 5 times/day of fruits and vegetables during the 7 days preceding the survey; 67.0% did not attend physical education classes daily; and 13.1% were overweight. Interpretation: Since 1991, the prevalence of many health-risk behaviors among high school students nationwide has decreased. However, many high school students continue to engage in behaviors that place them at risk for the leading causes of mortality and morbidity. The prevalence of many health-risk behaviors varies across cities and states. Public Health Action: YRBS data are used to measure progress toward achieving 15 national health objectives for Healthy People 2010 and three of the 10 leading health indicators, to assess trends in priority health-risk behaviors among high school students, and to evaluate the impact of broad school and community interventions at the national, state, and local levels. More effective school health programs and other policy and programmatic interventions are needed to reduce risk and improve health outcomes among youth. (Contains 1 box, 1 figure and 73 tables.) [For the 2003 report, "Youth Risk Behavior Surveillance--United States, 2003. Morbidity and Mortality Weekly Report. Surveillance Summaries. Volume 53, Number SS-2," see ED485659.]