A study of 1140 pregnancies ending in spontaneous abortion disclosed a central nervous system (CNS) malformation in 4.9% of all complete conceptuses. Life-table analysis suggested that the incidence of CNS malformations is 16/1000 at the beginning of the eighth week of gestation. It was also estimated that only one-fifth of these infants are born alive, 41% being aborted spontaneously and 38% stillborn. A hypothesis that differences in the incidence of CNS malformations result from area differences in the mortality rate of malformed embryos and fetuses was examined by comparing the findings in Northern Ireland, an area of high incidence, with those in south-east England, an area of low incidence. In Northern Ireland 4.6% of complete conceptuses had a CNS malformation compared with 3.0% in south-east England, but the difference was not statistically significant. There is no evidence that in Northern Ireland a lower mortality rate among malformed fetuses and embryos is responsible for the high incidence of malformation at birth. The geographical variation of CNS malformations in the United Kingdom still awaits explanation.