The Centers for Disease Control and Prevention's Youth Risk Behavior Survey (YRBS) has been used on a biennial basis since 1990 to measure health risk behaviors of high school students nationwide. The YRBS measures behaviors related to intentional and unintentional injury, tobacco use, alcohol and other drug use, sexual activity, diet, and physical activity. The authors present the results from a test-retest reliability study of the YRBS, conducted by administering the YRBS questionnaire to 1,679 students in grades 7 through 12 on two occasions 14 days apart. The authors computed a kappa statistic for each of 53 self-report items and compared group prevalence estimates across the two testing occasions. Kappas ranged from 14.5% to 91.1%; 71.7% of the items were rated as having "substantial" or higher reliability (kappa = 61-100%). No significant differences were found between the prevalence estimates at time 1 and time 2. Responses of seventh grade students were less consistent than those of students in higher grades, indicating that the YRBS is best suited for students in grade 8 and above. Except for a few suspect items, students appeared to report personal health risk behaviors reliably over time. Reliability and validity issues in health behavior assessment also are discussed.