We used confirmatory factor analysis to test whether a single factor might explain the clustering of the metabolic syndrome (MS) components in children.We studied 1,020 children aged 10-13 years from 20 schools in Cuenca, Spain. The single-factor model included: waist circumference (WC), fasting insulin, triglyceride to HDL cholesterol ratio (Triglyl/HDL-C), and mean arterial pressure (MAP). The standardized scores of the four variables in the model were used to develop a continuous MS index.Factor loadings were 0.67 for WC, 0.68 for fasting insulin, 0.57 for Triglyl/HDL-C, and 0.37 for MAP. The single-factor model also showed a good fit to the data. As compared with Adult Treatment Panel III criteria, the MS index showed strong validity in the diagnosis of MS (area under the receiver operating characteristic curve = 0.98, 95% CI 0.96-0.99).A single underlying factor has acceptable validity to represent MS in children.