Objective. We evaluated the presence of oxidative stress in obese children without co-morbidities. Methods. The study population included 68 children (30 girls, 38 boys), between 6 and 14 years of age. The levels of markers of oxidative damage (malondialdehyde [MDA], and plasma carbonyl groups [CG]) and measures of antioxidant defense, such as the enzyme glutathione peroxidase (GPx) and low molecular scavengers (erythrocyte-reduced glutathione [GSH], α-tocopherol and β-carotene) were determined. Children were categorized in groups by the standard deviation score of body mass index (SDS-BMI). Twenty children were non-obese (SDS-BMI≤1.33), and the 48 obese children (SDS-BMI≥2) were further divided into two groups: SDS-BMI≥3 (22 children) and ≥2 SDS-BMI<3 (26 children). Results. The levels of MDA and CG were significantly higher (p <0.05) in children with SDS-BMI≥3. The GPx activity was increased, while the GSH concentration was lower in obese children compared with non-obese children (p <0.01). There were no differences in serum α-tocopherol and β-carotene levels between groups. MDA was the sole marker of oxidative damage that was positively correlated with SDS-BMI, (r=0.35, p=0.015), and negatively with high-density lipoprotein cholesterol (HDL-C) (r=− 0.32, p=0.027). GPx was inversely related to total cholesterol (r=− 0.34, p=0.019). In multiple regression analysis, we confirmed that SDS-BMI and HDL-C were determinants of MDA. Conclusions. Severe childhood obesity is associated with oxidative stress. Thus, providing foods with high antioxidant capacity in addition to a hypocaloric diet is crucial for the treatment of obese children.