Folic acid is a compound that does not occur naturally in food but is added as a fortificant and dietary supplement. When it is ingested it is converted into forms of reduced folate that are identical to those arising from ingestion of naturally occurring folate in foods; however, some folic acid may appear unmetabolized in the serum (1, 2). Very little is known about its metabolism and biological effects. Folic acid fortification increased dietary intakes of folic acid (3) and blood folate levels in the United States (4). Some (5–9) but not all (10–12) research suggests that high folic acid intakes may promote the growth of pre-existing cancers or malignant lesions.