IN this issue of the Journal, Czeizel and Dudás¹ report from Hungary that vitamin supplements can prevent the occurrence of neural-tube defects (spina bifida and anencephaly). This report is a landmark in the definition of the requirements for vitamins and in the evolution of the role of scientific evidence in public health policy. It has long been suspected that dietary factors have a role in neural-tube closure; the possibility that folic acid might be involved was first proposed in the mid-1960s.² During the past decade our understanding of the relation between folic acid and birth defects has unfolded with . . .