Geographical variation in the incidence of neural tube abnormalities, particularly anencephalus, is well documented. Penrose (1957) reviewed the available European data and found that the highest rates for anencephalus were found in Ireland, Wales, and western regions of England and Scot land, and that they decreased progressively to wards Eastern Europe. Stevenson, Johnston, Stewart, and Golding (1966), in their comparative study of congenital malformations, confirmed a wide variation in the incidence of neural tube defects in the 24 centres studied. From national statistics (Fig. 1) it is apparent that the highest stillbirth rates in the British Isles due to neural tube abnormalities in the years 1961 1965 occurred in Northern Ireland, Scotland, and Wales and decreased progressively towards the south and east of England. An important question arising from these observ ations is whether the geographical variations in neural abnormalities noted above are related to environmental factors operating during pregnancy, or whether they may be due to other long-term influences, including the genetic structure of the population. A possible method of resolving this problem in polytypic populations, as shown by Carter and Woolf (1961) in a study of phenyl ketonuria, is to examine the parental birthplaces of cases to determine whether migrant groups share the experience of the host population or the popu lation from which they originated. Thus Carter and Woolf, by examining the birthplaces of parents and grandparents of children with phenyl ketonuria born in south-east England, were able to demonstrate that the gene frequency for this disorder is approxi mately four times greater in the population of Ireland and west Scotland than in south-east England. In this paper a study is made of the risk of neural tube defects occurring in babies born in the Oxford Record Linkage Study Area in the five