Agreement was sought among six indicators used to classify youth as obese in 625 white youth, aged 12.0–18.0 y, who participated in the Third National Health and Nutrition Examination Survey. Indicators included body mass index (BMI), triceps and subscapular skinfolds, the sum of four skinfolds, waist circumference and percentage body fat determined by bioelectric impedance analysis (BIA). The fattest youth in each age and gender group were considered those >80th centile for the indicator. Agreement was determined by kappa coefficients, which provide the chance-adjusted proportion of agreement in the upper quintiles to identify the same youth as the fattest. Kappas among indicators range from 0.57–0.85 for males and from 0.56–0.79 for females. Categorical agreement with the fattest youth by percentage body fat, changes considerably with age for most indicators, suggesting that relationships among indicators change during adolescence. Different indicators may identify different subpopulations as the fattest, arguing for caution in use and interpretation of results from different indicators, and in favour of standardized definitions for obesity in youth.