To determine in children the association between waist circumference (WC) and insulin resistance determined by homeostasis modeling (HOMA-IR) and proinsulinemia and components of the metabolic syndrome, including lipid profile and blood pressure (BP). Eighty-four students (40 boys) aged 6 to 13 years and matched for sex and age underwent anthropometric measurements; 40 were obese; 28, overweight; and 16, nonobese. Body mass index (BMI), WC, BP, and Tanner stage were determined. An oral glucose tolerance test, lipid profile, and insulin and proinsulin assays were performed. Children were classified as nonobese (BMI < 85th percentile), overweight (BMI, 85th-94th percentile), and obese (BMI > or = 95th percentile). There was univariate association (P < .01) between WC and height (r = 0.73), BMI (r = 0.96), Tanner stage (r = 0.67), age (r = 0.56), systolic BP (r = 0.64), diastolic BP (r = 0.61), high-density lipoprotein cholesterol level (r = 0.45), triglyceride level (r = 0.28), proinsulin level (r = 0.59), and HOMA-IR (r = 0.59). Multiple linear regression analysis using HOMA-IR as the dependent variable showed that WC (beta coefficient = 0.050 [95% confidence interval, 0.028 to 0.073]; P = .001) and systolic BP (beta coefficient = 0.033 [95% confidence interval, 0.004 to 0.062]; P = .004) were significant independent predictors for insulin resistance adjusted for diastolic BP, height, BMI, acanthosis nigricans, and high-density lipoprotein cholesterol level. Waist circumference is a predictor of insulin resistance syndrome in children and adolescents and could be included in clinical practice as a simple tool to help identify children at risk.