Noting that health-risk behaviors among youth may result in immediate health problems or extend into adulthood and increase risk for chronic diseases, this report examines the prevalence of health-risk behaviors among a nationally representative sample of persons aged 12 to 21 years and presents age group comparisons of the most important health-risk behaviors. Risk behaviors include failure to use car safety belts, possession of weapons, use of tobacco or alcohol, use of drugs, and engagement in sexual activity. The study used national data from the Youth Risk Behavior Survey (YRBS), which periodically measures the prevalence of priority health-risk behaviors among adolescents. The findings showed that at least one fourth of all 12- to 13-years-olds engage in one health-risk behavior, underscoring the importance of initiating prevention measures early and, because the prevalence of health-risk behaviors generally increases with age, of reinforcing such measures in later school years, including middle and senior high school. (AP)