Neural tube defects (NTDs) may result from a genetic susceptibility interacting with environmental exposures occurring early in pregnancy. Current research is concerned with enlarging our understanding of the action of folic acid, a B group vitamin, which has been shown to prevent the occurrence of NTDs in clinical trials. Despite the epidemic waves in the incidence of NTDs and the existence of areas with very high rates, there have been few studies that explored the genetic contribution to NTDs in high rates versus low rate areas. We investigated the genetic epidemiologic factors that occur in NTD families and compared their frequency in a high rate area-Ireland-with a low rate area-Italy. We explored the existence of three features indicative of hereditary factors and found that all three factors were higher in Ireland than in Italy. These factors were (i) sibling recurrence risk (3.3% vs 1.6%; p = 0.2), (ii), other malformations in siblings (11.5% vs 3.3%; p < 0.001) and (iii) average number of children in mothers' families vs fathers' families (average difference in Ireland 1.0 vs 0.4 in Italy; p < 0.1). These results support the motion that geographic differences in occurrence of NTDs are due at least in part to differing prevalences of genetic susceptibility factors. Further epidemiological and molecular studies are needed to confirm this observation. In addition, studies of the interactions between environmental agents and genetic susceptibility will be important in determining their relative contributions.