BackgroundChildhood obesity preventive interventions should promote a healthy diet and physical activity at home and school. This study aims to describe a school-based childhood obesity preventive programme (POIBA Project) targeting 8-to-12- year-olds.Design and methodsEvaluation study of a school-based intervention with a pre-post quasi-experimental design and a comparison group. Schools from disadvantaged neighbourhoods are oversampled. The intervention consists of 9 sessions, including 58 activities of a total duration between 9 and 13 hours, and the booster intervention of 2 sessions with 8 activities lasting 3 or 4 hours. They are multilevel (individual, family and school) and multicomponent (classroom, physical activity and family). Data are collected through anthropometric measurements, physical fitness tests and lifestyle surveys before and after the intervention and the booster intervention. In the intervention group, families complete two questionnaires about their children’s eating habits and physical activity. The outcome variable is the cumulative incidence rate of obesity, obtained from body mass index values and body fat assessed by triceps skinfold thickness. The independent variables are socio-demographic, contextual, eating habits, food frequency, intensity of physical activity and use of new technologies.Expected impact for public healthIt is essential to implement preventive interventions at early ages and to follow its effects over time. Interventions involving diet and physical activity are the most common, being the most effective setting the school. The POIBA Project intervenes in both the school and family setting and focuses on the most disadvantaged groups, in which obesity is most pronounced and difficult to prevent.Significance for public healthOverweight and obesity are a major public health concern that predispose affected individuals to the development of chronic diseases. Of importance, obesity is more common among disadvantaged neighbourhoods and it is precisely these environments that have the lowest involvement in preventive interventions. Consequently, it is very important to target these populations when designing an intervention. This study will provide an evaluated childhood obesity intervention which includes the perspective of gender and social inequalities. In addition, a sustainability plan has been developed for the intervention. It is planned that, in the near future, the intervention and its reinforcement are part of the set of health educational programmes offered by the Agència de Salut Pública de Barcelona (Spain) to schools, with funding both the cost of the training and the materials borne by the Agència.