In September 1992, the U.S. Public Health Service (PHS) recommended that women of childbearing age (i.e., aged 15-44 years) who are capable of becoming pregnant should consume 400 microg of the B-vitamin folic acid to reduce the number of cases of spina bifida and anencephaly (neural tube defects [NTDs]). Since then, an ongoing national effort has encouraged women to consume dietary supplements containing folic acid. In 1998, the Food and Drug Administration (FDA) required the fortification of enriched cereal grain products with folic acid, and manufacturers have voluntarily added more folic acid to many ready-to-eat breakfast cereals (CDC, unpublished data, 2002). To assess temporal changes in serum and red blood cell (RBC) folate concentrations among childbearing-aged women, CDC compared folate concentrations for childbearing-aged women who participated during 1988-1994 and 1999-2000 in the National Health and Nutrition Examination Survey (NHANES). This report outlines the results of this comparison and describes serum and RBC folate levels by race/ethnicity. The results indicate that over the period studied, the median serum folate concentration for women aged 15-44 years increased approximately threefold, and the median RBC folate concentration increased approximately twofold. These findings indicate that the national health objective for 2010 to increase the median RBC folate level among women of childbearing age to 220 ng/mL RBC (objective 16.16b) has been met for Mexican-American and non-Hispanic white women but not for non-Hispanic black women. To reduce the number of pregnancies affected by NTDs further, all women of childbearing age capable of becoming pregnant should consume the PHS-recommended level of folic acid daily.