BACKGROUND Childhood obesity is a major and increasing health problem for society because it increases the risk of cardiovascular disease, type 2 diabetes mellitus, and hypertension. Thus, when obese children become obese adults, effects on their health and life expectation may be devastating. OBJECTIVE 1) To assess the prevalence of metabolic syndrome (MS) in a child population with obesity. and 2). To compare anthropometric and biochemical parameters in patients with one or two parameters of MS syndrome to those of patients who meet MS criteria. METHODS A descriptive, cross-sectional study was conducted in children and adolescents with severe obesity (weight>p97) seen at the endocrinology department of Hospital de Getafe. Variables examined included age, sex, height, weight, body mass index (BMI),waist circumference(WC), oral glucose tolerance test (OGTT), insulin, insulin resistance (IR) measured by HOMA, triglycerides (TG), HDL, and systolic and diastolic blood pressure (SBP and DBP). The definition of MS in adolescents was made according to criteria of the International Diabetes Federation (IDF), 2007. RESULTS A total of 133 patients, 67 males (50.4%) and 66 females (49.6%) with a mean age of 12.17 ± 3.27 years, were enrolled into the study. All patients were obese, with a weight greater than the 97 h percentile for age and sex. Prevalence of several cardiovascular risk factors was as follows: WC ≥ 90th percentile for age and sex, 100%; hypertension, 26.08%; hypertriglyceridemia ≥ 150 mg/dL, 15.94%; HDL <40 mg/dL, 10.86%; fasting blood glucose levels ≥ 100mg/dL, 7.97%. The overall prevalence of metabolic syndrome was 19.6%. A comparison of different anthropometric and biochemical parameters in patients with 1 or 2 MS criteria to those with 3 or more criteria showed that obesity and insulin resistance were significantly greater the greater the number of MS criteria met. CONCLUSIONS 1.) Prevalence of MS in obese children and adolescents is high, 2). Arterial hypertension and hypertriglyceridemia are the most prevalent metabolic changes in the population studied and 3). Early intervention to control childhood obesity is essential to prevent cardiovascular morbidity and mortality in the future.