OBJECTIVES: To assess how outcomes associated with participation in a family-based weight management intervention (MEND 7-13, Mind, Exercise, Nutrition..Do it!) for childhood overweight or obesity implemented at scale in the community vary by child, family, neighbourhood and MEND programme characteristics. METHODS/SUBJECTS: Intervention evaluation using prospective service level data. Families (N=21,132) with overweight children are referred, or self-refer, to MEND. Families (participating child and one parent/carer) attend two sessions/week for 10 weeks (N=13,998; N=9563 with complete data from 1788 programmes across England). Sessions address diet and physical activity through education, skills training and motivational enhancement. MEND was shown to be effective in obese children in a randomised controlled trial (RCT). Outcomes were mean change in body mass index (BMI), age- and sex-standardised BMI (zBMI), self-esteem (Rosenberg scale) and psychological distress (Strengths and Difficulties Questionnaire) after the 10-week programme. Relationships between the outcome and covariates were tested in multilevel models adjusted for the outcome at baseline. RESULTS: After adjustment for covariates, BMI reduced by mean 0.76 kg m(-2) (s.e.=0.021, P<0.0001), zBMI reduced by mean 0.18 (s.e.=0.0038, P<0.0001), self-esteem score increased by 3.53 U  (s.e.=0.13, P<0.0001) and psychological distress score decreased by 2.65 U (s.e.=0.31, P<0.0001). Change in outcomes varied by participant, family, neighbourhood and programme factors. Generally, outcomes improved less among children from less advantaged backgrounds and in Asian compared with white children. BMI reduction under service conditions was slightly but not statistically significantly less than in the earlier RCT. CONCLUSIONS: The MEND intervention, when delivered at scale, is associated with improved BMI and psychosocial outcomes on average, but may work less well for some groups of children, and so has the potential to widen inequalities in these outcomes. Such public health interventions should be implemented to achieve sustained impact for all groups.