In the United States, approximately three-fourths of all deaths among persons aged 10-24 years result from only four causes: motor-vehicle crashes, other unintentional injuries, homicide, and suicide. Results from this 1999 national Youth Risk Behavior Survey demonstrate that numerous high school students engage in behaviors that increase the likelihood of their death from these four causes. During the 30 days preceding the survey, 16.4% had rarely or never worn a seat belt; 33.1% had ridden with a driver who had been drinking alcohol; 17.3% had carried a weapon; 50.0% had drunk alcohol; 26.7% had used marijuana; and during the 12 months preceding the survey 7.8% had attempted suicide. Substantial morbidity and social problems among young persons also result from unintended pregnancies and STDs, including HIV infection. Two-thirds of all deaths among persons under 25 years of age result from only 2 causes--cardiovascular disease and cancer. The majority of risk behaviors associated with these two causes of death are initiated during adolescence. Results from this study can be utilized by health and education officials to analyze and improve policies and programs that reduce health-risk behaviors among youth. (Contains 43 tables and 11 references.) (Author/MKA)