The rate and occurrence of childhood obesity is concerning and sets young people on a trajectory of lifetime physical health and psychological problems. Although randomized controlled trials have demonstrated the effectiveness of family-based pediatric lifestyle modification interventions, there is still a dearth of research evaluating behavioral and psychological predictors of treatment outcome under realworld practice settings.Objective: To address this gap, we explored demographic, behavioral, and psychological predictors of treatment success in a family-based pediatric weight management program in a hospital-based clinic.Methods: For 662 consecutively-treated children with overweight/obesity, we conducted four linear regressions examining demographic, psychological and behavioral predictors of post BMI z-scores (z-BMI), controlling for pretreatment BMI z-scores.Results: Overall there was a significant decrease in z-BMI from pre to post-treatment. Linear regressions revealed that participants who began treatment at a younger age, lower BMI (<99 th percentile), and who, at pre-treatment, were more likely to have externalizing problems (CBCL) had a lower z-BMI at post treatment, after controlling for pretreatment z-BMI. Results also indicated that those who attended more visits, specifically more visits with an exercise specialist, and who achieved more behavioral goals in treatment had a lower z-BMI at post treatment, after controlling for pre-treatment z-BMI. Conclusion:Our findings provide modest support for the effectiveness of family-based pediatric lifestyle modification interventions in hospital-based clinics. We suggest the need for health care providers to identify and prepare children and their families to enter treatment at a younger age and lower BMI, to emphasize sessions with exercise specialists, and treatment engagement/goal achievement at home.