OBJECTIVE : Obesity, obstructive sleep apnea (OSA), and type 2 diabetes mellitus (T2DM) are associated with chronic low-grade inflammation and oxidative stress. In adults, increased lipid peroxidation, a marker of oxidative stress, was found in both metabolic syndrome and OSA. Studies on oxidative stress in children with T2DM and OSA are scarce. METHODS : Plasma oxidized low-density lipoprotein (Ox-LDL) levels were evaluated in obese children and adolescents with/without T2DM, and the contribution of OSA to oxidative stress was investigated. RESULTS : Ten patients with T2DM, 8 with impaired glucose tolerance (IGT), and 20 body mass index-standard deviation score (BMI-SDS)-matched non-diabetic children (controls) were studied. They all underwent overnight polysomnography. Fasting plasma concentrations of Ox-LDL were measured and compared to the glycemic status and to the presence of OSA. Fourteen patients (36%) were diagnosed with OSA and 21 (55%) with hypertension. There were no significant group differences in plasma Ox-LDL levels or between patients with/without OSA. Plasma Ox-LDL levels were significantly higher among patients with hypertension compared to controls (P = 0.01), while they correlated with homeostasis model assessment (P = 0.02), BMI-SDS (P = 0.049), and systolic blood pressure (P = 0.002). CONCLUSIONS : The findings of this pilot study suggest that increased lipid peroxidation is associated with insulin resistance and hypertension in obese children and adolescents, while OSA has most likely minor influence.