AbstractContextChildhood obesity increases the risk of diseases as diabetes, cardiovascular disease, and nonalcoholic fatty liver disease.ObjectiveTo evaluate the prevalence of comorbidities in school-age children with obesity and to compare its prevalence and the effect of a lifestyle intervention between children in primary and secondary school and between boys and girls.DesignCross-sectional analysis and lifestyle intervention.SettingCentre for Overweight Adolescent and Children’s Healthcare.PatientsComorbidities were evaluated in 149 primary and 150 secondary school children with (morbid) obesity (162 girls). The effect of lifestyle intervention was studied in 82 primary and 75 secondary school children.InterventionOne-year interdisciplinary lifestyle intervention.ResultsInsulin resistance (37%), impaired glucose tolerance (IGT) (3%), dyslipidemia (48%), hypertension (7%), and elevated liver transaminase levels (54%) were already common in primary school children. Glomerular hyperfiltration and insulin resistance were more prevalent in secondary school children. IGT was more prevalent in girls. The change in body mass index z score after intervention was greater in primary school children (primary vs secondary: −0.25 ± 0.32 vs −0.11 ± 0.47), even as the change in low-density lipoprotein cholesterol concentrations [primary vs secondary: −0.30 (interquartile range, −0.70 to 0.10) vs −0.10 (interquartile range, −0.40 to 0.30)] and systolic blood pressure z score (primary vs secondary: −0.32 ± 1.27 vs 0.24 ± 1.3). The change in body mass index z score, but not in comorbidities, was greater in boys (boys vs girls: −0.33 ± 0.45 vs −0.05 ± 0.31).ConclusionsThe presence of comorbidities is already evident in primary school children with obesity. The effect of a lifestyle intervention on these comorbidities is greater in primary compared with secondary school children, stressing the need for early interventions.