The prevalence of the childhood obesity through the last decades is increasing, with significant comorbidities and major health consequences. Children's food habits are influenced by family, friends, school and the media. Successful interventions for preventing childhood obesity must include nutrition education, physical activity, school-based programs, family participation and counseling.To evaluate the prevalence of overweight and obesity among children and adolescents in an urban area in Lisbon and to implement a school-based program to prevent obesity.A cross sectional study was performed in children and adolescents attending the public school in an urban area of Lisbon. Learning sessions regarding obesity were performed.The study included 1875 children and adolescents, with ages between five and 17 years. The overall prevalence rate found for overweight and obesity was 21% and 9,5%, respectively.Obesity and overweight are important problems in children and adolescents. School-based programs can have an important role in preventing this disease.