Background: Family-based behavioural intervention programs (FBIPs) against childhood obesity have shown promising results, but the mediating mechanisms have not been identified. The aim of this study was to examine changes in obese childrens lifestyle habits during a 2-year FBIP according to their own and parents' reports, the concordance between these reports and the correlations to change in post-intervention z-BMI. Methods: An observational study of 26 children (8.3-12.0 years) and their parents participating in a 2-year FBIP was performed. Weight and height were measured from baseline to 12 months after the end of the program. Eating habits and physical-and sedentary activity were reported separately by children and parents. Data were analysed with regard to concordance between parents' and children's reports and association between the lifestyle reports and change in z-BMI at the study endpoint using descriptive statistics and parametric and non-parametric tests. Results: According to both children's and parents' reports, the level of physical activity among the children had increased after the intervention as well as the agreement between the informants' reports. According to the children, eating habits had improved, while the parents' reports showed an improvement only with regard to binge eating. The concordance between children and parents regarding eating habits was slight to fair also after the intervention. No statistically significant associations between changes in lifestyle reports and changes in z-BMI were observed. Conclusions: Child and parent reports of physical activity were found to converge and display an improvement in a 2-year FBIP, while the reports on eating habits showed a more refractory pattern. Changes in concordance and agreement between children and parents reports did not correlate with weight reduction. Further methods development and studies of the processes during family-based interventions against childhood obesity are warranted.