['To investigate the relationship among obesity, cardiovascular disease risk factors (CVDRFs), and plasma complement C3 concentration in children and adolescents.', 'In a nationwide survey conducted between 1992 and 2000, all school children aged 6-18 yr with abnormal results in repeated urine samples, including hematuria, proteinuria, and glucosuria (n = 97 312; 36 557 boys and 60 755 girls), were investigated for their body mass index (BMI), blood pressure, fasting plasma glucose, total cholesterol, and plasma complement C3 concentrations.', 'Children in the higher percentile groups for BMI or having more CVDRFs, namely, hypertension, diabetes, and hypercholesterolemia, had higher plasma C3 concentrations independently (p for both trends <0.05, adjusted for age and gender). The odds ratios (ORs) for having one, two, or three CVDRFs in obese children were 4.74 [95% confidence interval (CI) = 4.47-5.03], 19.8 (95% CI = 17.8-22.0), and 139 (95% CI = 96.6-200), respectively, adjusted for age, gender, and family history of diabetes, which were substantially reduced after adjustment for plasma C3 concentrations. The ORs for children with plasma C3 concentrations in the highest quartile to have one, two, or three CVDRFs were 2.32 (95% CI = 2.21-2.44), 5.68 (95% CI = 4.83-6.67), and 58.6 (95% CI = 19.7-174), respectively, adjusted for age, gender, family history of diabetes, and BMI.'] Are obesity and clustering of cardiovascular disease risk factors associated with elevated plasma complement C3 in children and adolescents?