Fortification of enriched cereal-grain products with folic acid to help prevent pregnancies affected by a neural tube defect (NTD) (e.g., spina bifida or anencephaly) became mandatory in the United States in January 1998. Data from the 1999-2000 National Health and Nutrition Examination Survey (NHANES) indicated that median serum folate concentrations in nonpregnant women of childbearing age had increased substantially, compared with concentrations during a period (1988-1994) before fortification was mandated. This report uses NHANES data to update those findings and assess trends in serum folate and red blood cell (RBC) folate levels by race/ethnicity from the 1999--2000 survey through the 2003-2004 survey. The results of these comparisons indicated that median serum folate concentrations among nonpregnant women of childbearing age decreased 16% from 1999-2000 through 2003-2004, and RBC folate concentrations decreased 8%. All women of childbearing age who are capable of becoming pregnant should consume 400 microg of folic acid daily to reduce the occurrence of NTD-affected pregnancies.