Children's energy balance-related behaviours (EBRB), comprising diet, screen time, physical activity, and sleep, combine into "lifestyle patterns", which may exert a synergistic effect on health. To date, studies investigating this synergy have primarily focused on obesity risk, without addressing other facets of health. To examinethe prospective associations of preschoolers' lifestyle patterns with socio-emotional, behavioural, and body mass index (BMI) outcomes at 8 years. Participants were 876 children from the EDEN mother-child cohort. Three lifestyle patterns (unhealthy, healthy, and mixed) were previously identified at age 5, separately in boys and girls. At age 8, height and weight measures generated BMI z-scores while social-emotional and behavioural development was assessed by parents using the Strengths and Difficulties Questionnaire (SDQ). Drawing from the outcome-wide approach, sex- and outcome-specific adjusted linear regressions were fitted. Boys' adherence to a healthy lifestyle pattern (combining a nutrient-dense diet and limited screen time) at 5 years was positively associated with prosocial behaviours (β= 0.14; 95% confidence interval [CI] 0.01, 0.26) and inversely related to hyperactivity-inattention symptoms (β= -0.12; 95% CI -0.23, -0.01) at 8 years. Girls' mixed lifestyle pattern (sugar or artificially sweetened beverages, high screen, physical activity and low sleep times) was associated with prosocial behaviours (β= 0.12; 95% CI 0.01, 0.23). There was no evidence of associations between lifestyle patterns and BMI z-scores. Findings suggest synergistic benefits of engaging in a combination of optimal EBRBs, especially in boys, and support intervention efforts at preschool age to enhance some dimensions of their later socio-emotional and behavioural development.