Mandatory fortification of enriched cereal-grain products became effective in the United States on January 1, 1998. This fortification was undertaken to assist women of child-bearing age in increasing their intake of folic acid to reduce their risk of having a pregnancy affected by a neural tube birth defect. The process by which the Food and Drug Administration modeled the level of fortification with folic acid illustrates the complex issues and general principles that emerge when fortification of a nation's food supply is evaluated as a means of addressing a public health concern. The effectiveness of fortification for a target population and safety for the much larger general population impose conflicting challenges that must be considered concurrently when making decisions regarding fortification. Recent data show improved folate status and apparent decreases in risk of neural tube birth defects in the U.S. Much about the long-term effects of the fortification program remains unknown and careful monitoring over time will be necessary to ensure that the program functions as intended.