INTRODUCTION: The objective of this study was to assess the impacts of a family-based childhood obesity treatment on anthropometry and predictors of dropout and successful weight loss. MATERIALS AND METHODS: The 18-month treatment consisted of a intensive period (IP) including physical exercise, nutritional guidance, family psychotherapy, child group sessions and a 1-year follow-up (FU). RESULTS: One hundred children (10-12 years old, >140% of median weight-for-height) participated. The 81 children completing the IP decreased significantly from 2.9 to 2.6 body mass index (BMI) standard deviation score (SDS) units (p < 0.001). Furthermore, mean body fat percentage decreased significantly from 33.7 to 31.9 during IP. The 49 children completing the FU had a further decrease of 0.2 BMI SDS units (p = 0.003). Weight loss was less in children from immigrant families. Drop-out was higher if the mother had limited education and in immigrant families. CONCLUSIONS: This treatment with a psychological approach is feasible and results in significant weight loss during the programme. Future research should focus on how to improve the results of families with limited education and immigrants with non-European origin.