Although the prevalence of childhood obesity, as assessed by BMI (kg/m(2)), has tripled over the last 3 decades, this index is a measure of excess weight rather than excess body fatness. In this review we focus on the relation of BMI to body fatness and health risks, particularly on the ability of BMI for age >or=95th Centers for Disease Control and Prevention CDC percentile to identify children who have excess body fatness. We also examine whether these associations differ according to race/ethnicity and whether skinfold and circumference measurements provide additional information on body fatness or health risks. The accuracy of BMI varies according to the degree of body fatness. Among relatively fat children, BMI is a good indicator of excess adiposity, but differences in the BMIs of relatively thin children can be largely due to fat-free mass. Although the accuracy of BMI in identifying children with excess body fatness depends on the chosen cut points, we have found that a high BMI-for-age has a moderately high (70%-80%) sensitivity and positive predictive value, along with a high specificity (95%). Children with a high BMI are much more likely to have adverse risk factor levels and to become obese adults than are thinner children. Skinfold thicknesses and the waist circumference may be useful in identifying children with moderately elevated levels of BMI (85th to 94 th percentiles) who truly have excess body fatness or adverse risk factor levels. A BMI for age at >or=95th percentile of the CDC reference population is a moderately sensitive and a specific indicator of excess adiposity among children.