Folate intake is inversely related to circulating total homocysteine (tHcy) concentrations. Elevated tHcy is a risk factor for vascular diseases. The objectives of this study were to present plasma tHcy distributions and investigate the association between tHcy and sex, age, and race-ethnicity in U.S. children and adolescents (3264 boys and 3197 girls) using data from the National Health and Nutrition Examination Surveys, 1999-2000 and 2001-2002 conducted in the post-folic acid fortification era. Plasma tHcy was higher in boys than in girls (P < 0.0001), and higher in older children (16-18 y old) than in younger children (3-15 y old) (P < 0.0001). The difference in plasma tHcy between boys and girls was greater in the 16- to 18-y-old group than in any other age group studied (P < 0.05). Age-adjusted plasma tHcy concentrations were approximately 6.8, approximately 10.5, and approximately 8.8% higher in boys than in girls in non-Hispanic White (NHW), non-Hispanic Black (NHB), and Mexican American/Hispanic (MA/H) children, respectively. Sex- and age-adjusted plasma tHcy concentrations (geometric means +/- SE) were 5.04 +/- 0.05, 5.01 +/- 0.06, and 4.99 +/- 0.06 micromol/L in the NHW, NHB, and MA/H groups, respectively. Race-ethnicity was not related to plasma tHcy in age-adjusted analysis for boys (P < 0.77) and girls (P < 0.26), and in sex- and age-adjusted analysis (P < 0.38) for all children. The plasma tHcy concentrations begin to rise between ages 8 and 11 y, and the age-related increase is greater in boys than in girls.