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Objective: To investigate the ability of BMI and waist circumference, single and combined, in identifying children who are at risk of hypertension and in influencing absolute blood pressure values., Methods: The body weight, height, waist circumference and blood pressure of 4177 5-11-year-old school children [2005 (48%) girls] were collected. Elevated blood pressure was defined if either systolic or diastolic blood pressure values or both were more than the 95th percentile according to sex, age and height (US normative blood pressure tables). Overweight and obese children were defined according to International Obesity Task Force BMI cut-offs., Results: The prevalence of hypertension was 4.1% and increased together with weight class: 1.4% (n = 42/3076) in normal weight, 7.1% (n = 59/827) in overweight and 25% (n = 69/274) in obese (P < 0.001). Only BMI and waist circumference showed a remarkable ability to discriminate hypertensive children (areas under receiver operating characteristic curves, 0.84 and 0.76, respectively). The multivariate analysis showed that z-scores for both BMI and waist circumference were significantly related to the risk of hypertension with odds ratios of 3.59 (95% confidence interval, 2.55, 5.06) and 1.20 (95% confidence interval, 1.04, 1.39), respectively, after adjusting for sex and age. When the weight class was included in the multivariate analysis, waist circumference retained its ability to identify hypertensive children only in the obese class (odds ratio, 1.44; 95% confidence interval, 1.21, 1.72; P < 0.01). When considering blood pressure as a continuous variable, both weight class and waist circumference showed a significant effect on systolic and diastolic blood pressure absolute values (P < 0.01). The effect of waist circumference on blood pressure values was maintained even when corrected for BMI., Conclusion: High blood pressure is strongly associated with excess weight. Waist circumference improves the ability of BMI to identify hypertension in obese children. Waist circumference is related to absolute blood pressure values in all weight classes.