To determine the prevalence and clinical phenotype of metabolic syndrome among overweight and obese schoolchildren in Beijing, and to compare the rates of diagnosis made according to the criteria of the National Cholesterol Education Program (NCEP) of the United States and International Diabetes Federation (IDF). Based on Beijing Child and Adolescent Metabolic Syndrome (BCAMS) study with body mass index (BMI), waist circumference (WC) and blood pressure measured, the overweight and obese children were screened among nearly 20 000 children 6-18 years of age in Beijing by Chinese BMI cutoffs for schoolchildren (7-18 years) and the US 2000 CDC Growth Charts--the 85th and 95th percentile (6 years) and were enrolled as the study population. Simultaneously a group of children with normal BMI were selected as the control group and based on the international method of age grouping, each of the above groups was divided further into 4 sub-groups in terms of age: 6-9, 10-12, 13-15 and 16-18 years old, respectively. Fasting plasma glucose (FPG) and insulin (FINS), serum high-density lipoprotein cholesterol (HDL-C) and triglyceride (TG) were examined. HOMA-IR index was calculated for estimating individual insulin resistance. A child who met any three or more of the following five criteria, according to NCEP definition, was diagnosed as MS. A diagnosis of MS using IDF definition required abdominal obesity plus any two or more of the other four criteria: (1) abnormal obesity: WC > or = P(90); (2) elevated BPs: SBP/DBP > or = P(90); (3) low HDL-C: HDL-C < 1.03 mmol/L (40 mg/dl); (4) high TG: TG > or = 1.24 mmol/L (110 mg/dl); (5) impaired fasting glucose (IFG): FPG > or = 5.6 mmol/L (100 mg/dl). The prevalence rates of MS by NCEP definition were: 0.9%, 7.6% and 29.8% in the normal weight (control group), overweight and obese children, respectively, which were higher than the rates diagnosed by IDF definition with 0.1%, 5.2% and 28.6% in the three groups. The prevalence rates of individual MS component among obese children were: 81.6% for abnormal obesity, 47.7% for elevated BPs, 35.6% for high TG, 16.9% for low HDL-C, and 13.4% for IFG. Elevated BPs (29.8%), abnormal obesity (27.4%) and high TG (26.0%) were the leading three abnormalities among overweight children. With the increase of BMI, the clustering of MS components and insulin resistance (HOMA-IR) were remarkably increased. HOMA-IR significantly increased as the number of MS component increased. MS has been in an epidemic status among the obese schoolchildren in Beijing. Abnormal obesity, elevated BPs and high TG were the three most common metabolic abnormalities for overweight and obese children. The prevalence rates of MS by NCEP definition in the present study was higher than those diagnosed by using IDF definition.