Objectives: Obesity prevention among children and adolescents is a public health priority; however, limited school-based intervention trials targeting obesity have been conducted. This article provides an overview of the study design and baseline preliminary findings of our ongoing school-based intervention study. Design: Randomized intervention trial to test a school-based, environmental obesity prevention program in urban low socioeconomic status (SES) African-American adolescents. The intervention program was developed based on several behavioral theories and was guided by preliminary findings based on focus group discussion and baseline data. Setting: Four Chicago public schools in the US. Subjects: Over 450 5 - 7th graders and their families and schools were involved. Results: Our baseline data indicate a high prevalence of overweight (43% in boys and 41% in girls) and a number of problems in these children's physical activity and eating patterns. Only 26% reported spending >= 20 min engaged in vigorous-moderate exercise in >= 5 days over the past 7 days; 29% reported spending >= 5 h each day watching TV, playing video games, or using computer. They also consumed too many fried foods and soft drinks. On average, 55% consumed fried foods >= 2 times/day over the past 7 days; regarding soft drinks, 70% reported consuming >= 2 times/day. Conclusion: School-based obesity prevention programs are urgently needed in the target US urban, low SES, minority communities. These data can be used to inform intervention activities.