This study aims to evaluate the effectiveness of Family-based behavioral social facilitation therapy (FBSFT) in the treatment of severe obesity in children and adolescents compared to treatment as usual (TAU). The FBSFT condition includes 17 weekly family-based treatment sessions at the hospital obesity clinic followed by monthly follow-up sessions with their local nurse and follow-up sessions at the hospital every third month for 2 years. The treatment targets both child and parent life-style; eating habits, physical activity, sedentary activity and sleep habits. Behavior modification techniques are employed; such as self-monitoring, goal setting, reward systems, problem solving and stimulus control. In addition, FBSFT focuses on facilitating lifestyle change across different settings (family, friends, school and community) and harnessing social support for healthy habits, which is considered important for long-term weight control. The TAU condition involves an assessment day with the multidisciplinary team (pediatrician, dietician, physical therapist and psychologist) at the hospital obesity clinic. Further a session with the nurse at the hospital clinic making a plan for behavioral lifestyle changes followed by monthly follow-up sessions with their local nurse and follow-up sessions at the hospital clinic every third month for one year.