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Background: Obesity is the main risk factor for essential hypertension (EH) in childhood. The O.Si.Me. study (Obesity and Metabolic Syndrome in children and adolescents) evaluated the prevalence of metabolic syndrome (MetS) and its constitutive traits in a sample of obese children and adolescents living in Campania, southern Italy. Patients and methods: Four hundred and fifteen children and adolescents consecutively referred to the National Health Service participating Outpatient Clinics for minor health problems and found to have a Body Mass Index (BMI) Z-score > 2.0 were enrolled in the study. The entire sample was screened for MetS, which was defined as the presence of at least 2 of the following alterations in addition to obesity: fasting hyperglycemia, low levels of high-density lipoproteins cholesterol, hypertriglyceridemia, and EH. The present analysis evaluated the clinical characteristics of the O.Si.Me subgroup of EH participants (systolic and/or diastolic BP ≥ 95 th percentile for age, gender and height) as compared with normotensive participants. Results: The prevalence of EH in the O.Si.Me population was 23.6 % (98/415, 48M and 50F.) and two-thirds of the EH participants met the MetS diagnostic criteria. The EH participants featured serum insulin and HOMA-IR levels significantly higher compared with normotensive ones (11.6±0.6 vs. 9.5±0.4 μIU/ml, p = 0.014; 2.6±0.1 vs. 2.2±0.1, p = 0.028 for insulin and HOMA-IR, respectively). These differences were common to boys and girls and remained significant after correction for age, pubertal stage, body weight, length, BMI, gestational age at birth, duration of breastfeeding and anthropometric parental parameters. Accordingly, children and adolescents with EH had a a relative risk of being insulin resistant (defined as a HOMA-IR ≥2.5) significantly greater compared to those without. Moreover, they exhibited higher serum creatinine levels (53.8±7.1 vs. 35.4±6.8 μmol/l, p=0.025) accounting for gender and body weight. Conclusions: More than a quarter of obese children and adolescents meet the diagnostic criteria for EH in the Campania region in southern Italy. These obese boys and girls have an increased prevalence of insulin resistance and apparently an initial reduction in renal function compared with obese children and adolescents with normal BP.