The geographical correlation between the incidence of spina bifida and anencephaly and the HLA and ABO antigen frequencies are studied. There is a positive correlation between these malformations and A1 and B8, and a negative correlation with B5 and Bw35. The role of the HLA system itself, or of a human T-like locus, is discussed. This study provides evidence of a possible genetic background of susceptibility to these malformations.