The prevalence of high body mass index (BMI) among children and adolescents in the United States appeared to plateau between 1999 and 2006. To provide the most recent estimates of high BMI among children and adolescents and high weight for recumbent length among infants and toddlers and to analyze trends in prevalence between 1999 and 2008. The National Health and Nutrition Examination Survey 2007-2008, a representative sample of the US population with measured heights and weights on 3281 children and adolescents (2 through 19 years of age) and 719 infants and toddlers (birth to 2 years of age). Prevalence of high weight for recumbent length (> or = 95th percentile of the Centers for Disease Control and Prevention growth charts) among infants and toddlers. Prevalence of high BMI among children and adolescents defined at 3 levels: BMI for age at or above the 97th percentile, at or above the 95th percentile, and at or above the 85th percentile of the BMI-for-age growth charts. Analyses of trends by age, sex, and race/ethnicity from 1999-2000 to 2007-2008. In 2007-2008, 9.5% of infants and toddlers (95% confidence interval [CI], 7.3%-11.7%) were at or above the 95th percentile of the weight-for-recumbent-length growth charts. Among children and adolescents aged 2 through 19 years, 11.9% (95% CI, 9.8%-13.9%) were at or above the 97th percentile of the BMI-for-age growth charts; 16.9% (95% CI, 14.1%-19.6%) were at or above the 95th percentile; and 31.7% (95% CI, 29.2%-34.1%) were at or above the 85th percentile of BMI for age. Prevalence estimates differed by age and by race/ethnic group. Trend analyses indicate no significant trend between 1999-2000 and 2007-2008 except at the highest BMI cut point (BMI for age > or = 97th percentile) among all 6- through 19-year-old boys (odds ratio [OR], 1.52; 95% CI, 1.17-2.01) and among non-Hispanic white boys of the same age (OR, 1.87; 95% CI, 1.22-2.94). No statistically significant linear trends in high weight for recumbent length or high BMI were found over the time periods 1999-2000, 2001-2002, 2003-2004, 2005-2006, and 2007-2008 among girls and boys except among the very heaviest 6- through 19-year-old boys.