OBJECTIVE: To describe the demographic characteristics of adolescent boys and girls who engage in three sedentary behaviors (television/video use, computer use, and reading/homework), and to explore how each sedentary activity is associated with body mass index (BMI), dietary behaviors, and leisure time physical activity. DESIGN: This study draws on data collected from Project EAT (Eating Among Teens), a school-based survey examining personal, behavioral, and socioenvironmental factors that are associated with nutritional intake among adolescents. SUBJECTS: The study sample consists of 4746 middle and high school students from 31 public schools in a metropolitan area of the upper Midwest. All students were invited to participate. The overall response rate for Project EAT was 81.5%. Data collection was completed during the 1998-1999 school year. STATISTICAL ANALYSIS: Multivariate linear regression was used for examining associations between independent and dependent variables, controlling for age, race/ethnicity, and socioeconomic status. All differences were considered statistically significant at P<.05. RESULTS: Among boys, television/video use and time spent reading/doing homework were positively associated with BMI (P<.05), whereas for girls television/video and computer use were positively associated with BMI (P<.05). High television/video use among boys and girls was associated with more unhealthful dietary behaviors (eg, increased consumption of soft drinks, fried foods, and snacks) (P<.05). In contrast, time spent reading/doing homework was associated with more healthful dietary behaviors (eg, increased consumption of fruits and vegetables) (P<.05). Leisure time physical activity was not associated with television/video use among boys or girls, but was positively associated with computer use and time spent reading/doing homework (P<.05). Applications/Conclusions Messages and advice aimed at reducing time spent in sedentary activities should be targeted at television/video use instead of time spent reading, doing homework, or using a computer. Nutrition education should incorporate messages about the influence of the media and advertising on dietary behaviors.