The goal of this study, based on information collected from eight congenital malformation registry programs around the world, was to analyze sex and twinning of affected infants and fetuses, according to type of neural tube defect (NTD) and other variables. The type of defect and its location, whether it occurred in isolated form or was associated with other malformations, population from which data were retrieved, vital status of the infant, and gestational age of the fetus/infant were considered. The material included 3,416 infants and 168 selectively aborted fetuses with anencephaly, 4,830 infants and 76 fetuses with spina bifida, and 1,022 infants and 19 fetuses with encephalocele. Less than 20% of infants with anencephaly or spina bifida and more than one-third of infants with encephalocele had associated malformations. A female excess was found among all infants with NTD but a male excess in fetuses delivered spontaneously before week 20, indicating selective male late fetal deaths. Sex ratio varied by type of NTD, vital status at birth, presence of associated malformations, and year of birth, with an increasing sex ratio for anencephaly during the 1960s and 1970s. Twinning was positively associated with NTD but the extent of this association varied with NTD type and program, while the distribution by sex type of pair of twin appeared to be similar to that of all births. Twins concordant for anencephaly or encephalocele were mainly found when the defect occurred as part of a syndrome, and only in like-sexed pairs. Twins concordant for spina bifida had the isolated form of the defect and 5 of 6 pairs were like-sexed.(ABSTRACT TRUNCATED AT 250 WORDS)