Neural tube defects--including spina bifida, anencephaly, and encephalocele--are common, serious birth defects that are important causes of infant mortality and disability. Women in the United States who have had a pregnancy resulting in an infant or fetus with a neural tube defect have a 2%-3% risk for having another pregnancy resulting in an infant or fetus with a neural tube defect (i.e., a recurrence) The British Medical Research Council (MRC) Vitamin Study Group recently reported the results of a randomized prevention trial that indicated that daily oral supplementation with folic acid before conception and during early pregnancy substantially reduces the recurrence of neural tube defects. This report summarizes the findings of that study and provides recommendations for supplementation with folic acid to prevent the recurrence of neural tube defects.