BACKGROUND: Unhealthy lifestyles contribute to the development of cardiovascular risk factors, whose incidence is increasing among children and adolescents. The Program SI! is a long-term, multi-target behavioral intervention to promote healthy lifestyle habits in children through the school environment. The objective of the study is to evaluate the efficacy of this intervention in its first phase, preschoolers. METHODS: Cluster-randomized controlled trial in public schools in the city of Madrid, Spain. A total 24 schools, including 2062 children (3-5 years), 1949 families, and 125 teachers participated in the study. Schools were assigned to their usual school curriculum or to engage in an additional multi-component intervention (Program SI!). The primary outcome of this trial is 1-school year changes from baseline in scores for children's knowledge, attitudes and habits (KAH). Secondary outcomes are 1-school year changes from baseline in scores for knowledge, attitudes, and habits among parents, teachers, and the school environment. RESULTS: After 1-school year, our results indicate that the Program SI! intervention increases children's KAH scores, both overall (3.45, 95% CI, 1.84-5.05) and component-specific (Diet: 0.93, 95% CI, 0.12-1.75; Physical activity: 1.93, 95% CI, 1.17-2.69; Human body: 0.65, 95% CI, 0.07-1.24) score. CONCLUSIONS: The Program SI! is demonstrated as an effective and feasible strategy for increasing knowledge and improving lifestyle attitudes and habits among very young children. TRIAL REGISTRATION: NCT01579708, Evaluation of the Program SI! for Preschool Education: A School-Based Randomized Controlled Trial (Preschool-SI!).