A total of 509 specimens of spontaneous abortion were studied. Of 364 complete specimens, 15 (4.1%) had central nervous system (CNS) abnormalities. The defects which were seen were anencephaly, spina bifida, iniencephaly, encephalocele and anencephaly combined with complete rachischisis. A high proportion (five out of 15) had the more unusual defects of iniencephaly and encephalocele. All except one had ceased development at less than 12 weeks of gestation. Sex chromatin studies showed that males outnumbered females amount CNS defective fetuses. When the month of conception was calculated for these abnormal pregnancies, summer and winter peaks were detected. Only one of the 15 patients had a family history of neural tube defect (NTD). Histological examination of all small fetuses aborted spontaneously revealed two additional facts. First, in fetuses with a localised external CNS defect, the internal abnormality was more extensive. Second, the CNS was also abnormal in some fetuses with no visible external defect. The proportion of abnormality is much higher than at birth and is also higher than in other surveys of spontaneous abortion. We suggest that screening for serum alphafetoprotein should be undertaken and amniocentesis considered in pregnancies subsequent to an abortion of this type.