Background: Previous studies on physical activity interventions in preschools have reported limited effectiveness. Participatory community-based approaches hold promise for increasing intervention effectiveness and involving parents as key stakeholders in a sustainable way. Purpose: To assess whether a participatory parent-focused approach using parents as agents of behavioral change enhances the efficacy of a preschool physical activity (PA) intervention. Design: Two-armed, cluster-RCT with preschool as unit of randomization and children as unit of analysis. Setting/participants: 39 South German preschools applying for an existing state-sponsored PA program with 826 children (52% boys, aged 5.0 +/- 0.2 years), with 441 allocated to the intervention arm. Intervention: Control preschools received a state-sponsored program consisting of twice-weekly gym classes over 6 months. In intervention preschools, this program was augmented by motivating parents to develop and implement their own project ideas for promoting children's PA. Main outcome measures: Primary outcomes included mean accelerometry counts and time spent in moderate- to vigorous-intensity PA or sedentary behavior. Secondary outcomes were BMI, percentage body fat, quality of life, sleep quality, and general health. Outcomes were measured at baseline and at 6 and 12 months in both study arms (time period: 2008-2010). Using an intention-to-treat-analysis, linear multilevel regression models assessed change over time and across study arms, adjusted for age, gender, season, and preschool location. Analysis was conducted in 2011. Results: In 15 intervention preschools, parents implemented 25 PA projects. Compared with controls, intervention arm children were 11 minutes less sedentary per day (95% CI=5.39, 17.01, p=0.014); had significantly more mean accelerometry counts (1.4 counts/15 seconds [95% CI=0.22, 2.54], p=0.019); and showed benefits in perceived general health and quality of life. All other outcomes showed no difference between study arms. Conclusions: A participatory preschool intervention focusing on parents as agents of behavioral change may be able to promote PA and reduce sedentary behavior in preschoolers. These benefits may go beyond the effects of existing nonparticipatory interventions. (author abstract)