Background: 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. Objectives: To examine the prospective associations of preschoolers' lifestyle patterns with socio-emotional, behavioural, and body mass index (BMI) outcomes at 8 years. Methods: 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. Results: 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. Conclusions: 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.