Although inpatient lifestyle treatment for obese children and adolescents can be highly effective in the short term, long-term results are unconvincing. One possible explanation might be that the treatment takes place far from parents' homes, limiting the possibility to incorporate the parents, who play a major role in establishing and maintaining a healthy lifestyle in childhood and adolescence. The main goal was to develop a brief behaviorally oriented parent training program that enhances ‘obesity-specific’ parenting skills in order to prevent relapse. We hypothesized that the inclusion of additional parent training would lead to an improved long-term weight course of obese children. Parents of obese children (n = 686; 7–13 years old) either participated in complementary cognitive-behavioral group sessions (n = 336) or received written information only (n = 350) during the inpatient stay. Children of both groups attended multidisciplinary inpatient rehabilitation. BMI-SDS as a primary outcome was evaluated at baseline, post-intervention and at 6- and 12-month follow-up. Intention-to-treat (ITT) as well as per-protocol analyses (PPA) were performed. A significant within-group decrease of 0.24 (95% CI 0.18 to 0.30) BMI-SDS points from the beginning of the inpatient stay through the first year was found, but no group difference at the one-year follow-up (mean difference 0.02; 95% CI -0.04 to 0.07). We also observed an increase in quality of life scores, intake of healthy food and exercise for both groups, without differences between groups (ITT and PPA). Thus, while the inpatient treatment proved highly effective, additional parent training did not lead to better results in long-term weight maintenance or to better psychosocial well-being compared to written psycho-educational material. Further research should focus on subgroups to answer the question of differential treatment effects.