The aim of this study was to investigate the frequency of metabolic syndrome (MS) variables in a group of spanish obese children and adolescents, to asses MS prevalence in this population and to describe it's relationship with other metabolic risk factors. 103 children were studied : 54 male and 49 female, mean age 10.08+/-2.3 with exogenous obesity. Obesity was defined when BMI was higher than the age and sex specific equivalent to 30 kg/m(2) in adults. MS variables considered were waist circumference, blood pressure, fasting blood triglycerides, fasting glucose/insulin and HDL-cholesterol. The children were considered as having the MS when three or more characteristics showed abnormal values according to Cook and De Ferranti definitions. HOMA index, ApoB and ApoA1 were studied too. The most frequent features of the metabolic syndrome were excess waist circumference and hypertension. The MS markers with the lowest frequency were dyslipidemia and fasting hyperglicemia. MS prevalence was 29,9% (Cook et al. criteria) and 50% (De Ferranti et al. criteria). Fasting insulin and HOMA index values increased significantly (p < 0.05) when three or more abnormalities of the MS variables were present. Apo B increased significantly only in females (p < 0.05) and Apo Al decreased significantly (p < 0.05) in both sexes when MS was present. Adequate metabolic syndrome risk factors criteria, mainly cut-off values, need to be defined in the European paediatric population.