The sex ratio of 147 fetuses with presumed multifactorial neural tube defect (NTD) was studied. Overall, the ratio (males:females) was 0.73 with expected female excess. However, when the NTDs were subdivided according to the site of the lesion, the sex ratios varied. Total craniorachischisis, anencephaly with cervical spina bifida, holoacrania, and thoracic spina bifida showed a greater female excess than that overall; the sex ratio for meroacrania was close to unity, while that for low spinal lesions, particularly those involving the sacrum, showed an extreme bias towards males. These findings are related to the mode of formation of the neural tube. Females seem prone to defects of neurulation and males to defects in canalization. An explanation for these findings is suggested in possible sex differences in rate of early embryonic development.