To describe and provide estimates of the distribution of waist circumference (WC) according to percentiles in African-, European-, and Mexican-American children, and to test for group differences at different percentiles. Cross-sectional data from the Third National Health and Nutrition Examination Survey (NHANES III) were examined. The sample evaluated included 9713 nonpregnant persons 2 to 18 years of age with measured values of WCs. Age-, sex-, and ethnicity-specific percentiles were estimated via percentile regression. WC measurements increased in a monotonic fashion across ages but at nonconstant rates and in a manner that varied across age and sex. At higher percentiles of the distribution, estimates of WC differ between Mexican-American (MA) and European-American (EA) and between African-American (AA) and European-American (EA), and, in some cases, exceeded the adult cutoff value for obesity-related disease risk at as early as 13 years of age. Age-, sex-, and ethnicity-specific WC percentiles are available for US children and adolescents and can be used as an assessment tool that could impact public health recommendations. Results suggest concern with respect to high WC values among certain ethnic groups.