Despite extensive research, little progress has been made in elucidating the etiologies of anencephalus and spina bifida. International and national distributions of disease occurrence have often been used as a basis for generating etiological hypotheses (e.g., potato blight, tea consumption, and zinc deficiency hypotheses). However, few of the epidemiological studies of neural tube defects (NTDs) have been conducted with scientific rigor in design, and most are of dubious validity, often with low precision in the estimates. This paper shows that the accepted geographic patterns of NTDs may be attributable to variations in the validity of studies used to describe these patterns. The nonuniformity in the duration and diligence of case ascertainment, the lack of a standardized nomenclature and classification, and the definition of the denominator remain principal problems in evaluating the epidemiology of NTDs. For example, the distinction between incidence and prevalence is not always made, and there is no consistency in the placement of the gestational boundary between late fetal deaths and spontaneous abortions. Findings are compared from studies conducted at different times, without due regard to the effect of secular trends, and using studies that have varying levels of case ascertainment. In etiological research, it is important to perform studies that are accurate and precise, but the literature used to define the spatial distribution of NTDs has often been accepted without due regard to the effect of these factors.