During the Child and Adolescent Trial for Cardiovascular Health (CATCH) Phase 3, a cross-sectional study of serum tHcy levels was conducted in eighth grade students (Jan-June 1997). Higher mean levels of plasma total homocysteine were observed among males, Blacks, and non-users of multi-vitamins and a strong, inverse association with serum levels of folic acid and to a lesser extent, with serum vitamin Bl2. A second measurement of the cohort at grade 12 was conducted in order to assess the dose-response relation between serum tHcy and dietary intakes (not measured in grade 8) of folic acid, vitamin B12 and vitamin B6. In addition and of equal importance, changes were evaluated in serum folic acid and tHcy levels from grade 8 to 12, after full implementation of fortification of cereal and grain products in the U.S. with folic acid and its impact on the distribution of tHcy levels in adolescents described. Effective January 1, 1998, the U.S. Department of Agriculture mandated the addition of folic acid to all flour and grain products in the United States. CATCH had a unique opportunity to examine the effect of "this natural experiment" on the distribution of serum tHcy in the cohort, because serum tHcy levels were measured in grade 8, just prior to full implementation of the mandate. Furthermore, the study had adequate sample size to examine these changes among important demographic subgroups such as, males vs. females and Caucasians vs. African Americans vs. Hispanics. Information generated by this study will be valuable for designing specific dietary interventions for youth and targeting subgroups of children who may be at higher risk for CVD.