Folate functions in multiple coenzyme forms in acceptance, redox processing and transfer of one-carbon units, including nucleotides and certain amino acids. Folate-requiring metabolic processes are influenced by folate intake, intake of other essential nutrients, including vitamins B-12 and B-6, and at least one common genetic polymorphism. Estimates of folate requirements have been based on intakes associated with maintenance of normal plasma and erythrocyte folate concentrations and functional tests that reflect abnormalities in folate-dependent reactions. Dietary Reference Intakes for folate that have been developed recently are based primarily on metabolic studies in which erythrocyte folate concentration was considered the major indicator of adequacy. For adults >/=19 y, the Recommended Dietary Allowance (RDA) is 400 microg/d of dietary folate equivalents (DFE); for lactating and pregnant women, the RDAs include an additional 100 and 200 microg of DFE/d, respectively.