Background: Intensive inpatient lifestyle treatment may be a suitable alternative for severely obese children and adolescents who do not benefit from ambulatory obesity treatment. The aim was to evaluate the effectiveness of two intensive one-year lifestyle treatments with varying inpatient periods for severely obese children and adolescents with regard to SDS-BMI and cardiometabolic risk factors. Methods: The study was designed as a randomized controlled trial with two active treatment groups. Eighty participants (8-19 years) with severe obesity received treatment at a specialized childhood obesity center in the Netherlands. Severe obesity was defined as a SDS-BMI >= 3.0 or a SDS-BMI >= 2.3 in combination with obesity-related comorbidity. Participants received an intensive one-year lifestyle treatment with an inpatient period of either two months and biweekly return visits during the next four months (short-stay group) or six months (long-stay group), both followed by six monthly return visits. Outcomes were assessed at baseline, six and 12 months and included SDS-BMI as primary outcome and cardiometabolic risk factors such as SDS-waist circumference, systolic-and diastolic blood pressure, and blood measurements as secondary outcomes. To evaluate differences in the course of the primary-and secondary outcomes over time between the two treatment groups, Generalized Estimating Equations (GEE) were performed. Results: No differences in the course of SDS-BMI or secondary outcomes over time were found between the two treatment groups after one year of treatment. SDS-BMI decreased statistically significantly after one year of treatment compared with baseline in both groups (0.33 (0.48) in the short-stay and 0.52 (0.49) in the long-stay group). Similar results were found for SDS-waist circumference, diastolic blood pressure and HDL-cholesterol. Conclusions: Since there were no significant differences in effects between the short-and long-stay treatment and considering the burden of the long-stay treatment for children and families, we recommend implementation of the short-stay treatment.