Mental health issues in children and young people are frequent and can have enduring negative consequences. Preventive early interventions delivered at school may foster psychological well-being, and preliminary evidence suggests that mindfulness-based social and emotional study was to evaluate a mindfulness-based SEL curriculum including a French adaptation of the Kindness Curriculum (KC), delivered by pre-kindergarten teachers, in a cluster randomized control trial. Sixty-four classes (761 children, aged 38-58 months) from a socioeconomically disadvantaged area in France were randomly assigned to either intervention or waiting-list control conditions. Indicator-management, positive relationships with teachers and peers, emotional processing and executive functioning were collected through teacher-rated questionnaires, standardized observations, and behavioral tasks. Results in mental health, including a reduction in emotional, conduct and peer relationship problems, and a reduction in teacher-child con icts. No signi cant effects were found on the other indicators. Heterogeneity analyses revealed stronger effects of the intervention on mental health and positive effects on self-management, relationships with teachers and relationships with peers in several subgroups. This program appears as a promising early intervention promoting mental health and positive relationships, especially for at-risk subgroups of preschool-aged children.