Objective. To determine baseline predictors of treatment success in terms of Body Mass Index-Standard Deviation Scores (BMI-SDS) in a multidisciplinary family-based behavioural lifestyle intervention for overweight and obese children. Methods. Overweight and obese children (N = 248; age 814 years) and their caregivers participated in a prospective study and attended a lifestyle intervention. Baseline data assessment included anthropometrics, demographics, breakfast behaviour, competence and behavioural problems (Child Behaviour Checklist [CBCL]), family functioning (Family Adaptability and Cohesion Evaluation Scales [FACES] III), and personality (Dutch Personality Questionnaire-Youth [NPV-J]). BMI-SDS was measured at start and after 3, 9, and 12 months of treatment. Mixed modelling was used for analysis. Results. Greater BMI-SDS reductions over the course of one year were found in children with Caucasian parents, with lower baseline BMI-SDS, and higher CBCL-social competence scores. Furthermore, children with non-overweight parents, younger children, and children with lower CBCL-somatic scores were more successful in BMI-SDS reduction. No effects on treatment success were found for the number or position of siblings, having divorced parents or a working mother, educational level of the parents, breakfast behaviour, family functioning, and personality. Conclusions. These results suggest that screening for baseline characteristics in childhood obesity treatment could identify who will benefit most from a paediatric lifestyle intervention. Tailored programs should be developed and the treatment team should focus on children who are less successful in achieving weight reductions. Future research should study by which mechanisms somatic complaints and social competence influence treatment success. © 2011 Informa Healthcare.