IntroductionPreventive interventions to reduce overweight and obesity in childhood and adolescence are studied on their effectiveness worldwide. A number with positive results. However, long-term effects of these interventions and their potentially wider influence on well-being and health have been less studied. This study aims to evaluate the long-term effects of a multicomponent intervention in elementary school children targeting individual behaviour as well as environment (Lekker Fit!). The primary outcomeis body mass index and the secondary outcomes are waist circumference, weight status, physical fitness, lifestyle, psychosocial health and academic performance.Methods and analysisIn a naturalistic effect evaluation with a retrospective, controlled design adolescents in secondary schools, from intervention and non-intervention elementary schools, will be compared on a wide set of outcome variables. Data will be collected by questionnaires and through anthropometric and fitness measurements by trained physical education teachers and research assistants. Baseline data consist of measurements from the adolescents at the age of 5 years old and are gathered from preventive youth healthcare records, from before the intervention took place. Multilevel regression models will be used and adjusted for baseline measurements and potential confounding variables on the individual and environmental level. Furthermore, propensity scores will be applied.Ethics and disseminationThe study has been approved by the Medical Research Ethics Committee of the Erasmus Medical Centre, Rotterdam, The Netherlands (permission ID: MEC-2020-0644). Study findings will be disseminated in peer-reviewed journals and by conference presentations.Trial registration numberNL8799. Pre-results.