The terms spina bifida and anencephaly include a range of developmental malformations that result from abnormal or incomplete closure of the neural tube. Despite advances in treatment and prenatal detection, these conditions remain as one of the most common and serious groups of birth defects. Spina bifida is associated with both increased mortality and morbidity, and anencephaly is always fatal. The occurrence of these conditions has a profound influence on affected individuals and their families and important public health implications. The etiology of NTDs has been of considerable interest for several decades. They are known to be etiologically heterogeneous and to occur in association with chromosome abnormalities, teratogenic exposures, and occasionally as part of single gene disorders. However, a specific causative agent cannot be identified in the vast majority of affected individuals. The etiology of NTDs in these "non-syndromic" patients is believed to be complex and to involve both genetic and environmental risk factors. Using a comprehensive research program, this study will evaluate the potential genetic determinants of SB and anencephaly in a large, well-characterized sample.