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2060 www. thelancet. com Vol 369 June 23, 2007 globally. This need prompted the IDF to develop a new simple definition. Consistent with criteria for adults, 2 this definition is a starting point that can be changed as information emerges. The new IDF definition for the metabolic syndrome in young people builds on previous studies that used modified adult criteria to investigate prevalence in children and adolescents (panel). 6–10 Thus a study of adolescents that used modified Adult Treatment Panel III (ATP III) criteria identified 12% with metabolic syndrome. 10 In those aged 12–19 years in the National Health And Nutrition Examination Survey III, 6 which used modified ATP-III criteria, about 10% had the syndrome. A recent study11 suggested a new set of criteria with age-specific and sex-specific cutoffs, which underlined the need for a consistent definition. Many variables are used to define obesity in children. However, waist circumference in children, consistent with the situation in adults, is an independent predictor of insulin resistance, lipid levels, and blood pressure. 7, 12 More over, in young people who are obese and have similar body-mass index, insulin sensitivity is lower in those with high amounts of visceral adipose tissue than in those with low amounts. 12 Therefore waist measurement is included in the new definition. Percentiles, rather than absolute values, of waist circumference have been used in the new definition to compensate for variation in child development and ethnic origin. Data for percentiles of waist circumference that are specific to ethnic origin are becoming increasingly available. 13 Children who have a waist circumference higher than the 90th percentile are more likely to have multiple risk factors for cardiovascular disease than are those with lower waist circumference. 14 Several studies6, 7, 15 have used the 90th percentile as a cutoff for waist circumference. These data, and the unequivocal evidence for the dangers of abdominal obesity in adults, lend support to its use as the main and essential component of the new definition for diagnosis of the metabolic syndrome in children and adolescents. The IDF has chosen to use the 90th percentile as a cutoff for waist circumference, which will be reassessed when more data are available. The new IDF definition is divided according to age-groups because of developmental challenges presented by age-related differences in children and adolescents: age 6 years to younger than 10 years; age 10 years to younger than 16 years; and 16 years or older. Children who are younger than 6 years were excluded from the definition because of insufficient data for this age-group. We suggest that the metabolic syndrome should not be diagnosed in children younger than age 10 years, but that a strong message for weight reduction should be delivered for those with abdominal obesity. For children aged 10 years or older, metabolic syndrome can be diagnosed by abdominal obesity and the presence of two or more other clinical features (ie, elevated triglycerides, low HDL-cholesterol, high blood pressure, or increased plasma glucose). Whereas one definition, albeit with cutoffs specific for sex and ethnic origin, is suitable for use in at-risk adults, 2 use of one definition in children and adolescents is problematic. Blood pressure, lipid levels, insulin sensitivity, and anthropometric variables change with age and pubertal development. However, in the absence of contemporary definitive data, the criteria adhere to the absolute values in the IDF adult definition, except that waist circumference percentiles are recommended and one (rather than a sex-specific) cutoff is used for HDL. For children …