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 4,746 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 analyses 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.