Although randomized controlled trials demonstrated the long-term efficacy of lifestyle interventions in overweight children, the effects of these interventions in clinical practice under real-life conditions are largely unknown. One hundred twenty-nine centers specialized in outpatient pediatric obesity care participated in this quality assessment. All patients presenting before the year 2006 for lifestyle intervention of at least 6 months duration in these institutions were analyzed in a 2-year follow-up. A total of 21,784 (45% male) overweight children and adolescents aged 2-20 years (mean BMI 30.4 kg/m2, mean SDS-BMI 2.51, mean age 12.6 years) were included in the analysis. Based on an intention-to-treat analysis with variables set back to baseline in lost of follow-up, 22% of the children reduced their SDS-BMI after 6 months, 15% after 12 months, and 7% after 24 months, but only in 24, 17, and 8% of children, respectively, complete data were available. In the five treatment centers with the best outcome (518 patients), 83% of the children reduced their overweight after 6 months, 67% after 12 months, and 51% after 24 months. Under real-life conditions, most treatment centers cannot prove the long-term efficacy of their interventions due to high drop-out rate or lack of documentation. Conversely, some institutions achieved a reduction of overweight in nearly the half of their patients 24 months after baseline demonstrating the great heterogeneity in outcome. To improve the effectiveness of lifestyle interventions in real-life studying, the process and structure quality as well as their long-term results is urgently needed.