In the United States, 70.8% of all deaths among youth and young adults aged 10-24 years result from only four causes: motor-vehicle crashes (32.3%), other unintentional injuries (11.7%), homicide (15.1%), and suicide (11.7%). Substantial morbidity and social problems also result from the approximately 870,000 pregnancies that occur each year among females aged 15-19 years and the estimated 3 million cases of sexually transmitted diseases (STDs) that occur each year among persons aged 10-19 years. This report summarizes results from the 2003 national school-based survey and trends during 1991-2003 in selected risk behaviors. Data from the 32 state and 18 of the 20 local school-based surveys with weighted data in 2003 also are included. Data from the remaining 11 state and 2 local school-based surveys with unweighted data are not included in this report. The national survey, the 32 weighted state surveys, and 15 of the weighted local surveys were conducted during spring 2003. Three of the local surveys were conducted during fall 2003. (Contains 66 tables and 8 figures.)