Detailed investigations were made on 150 fetuses with neural tube defects (NTD). After eliminating those with recognised causes, the rest were found to consist of fetuses with both isolated NTD and NTD with other developmental abnormalities. On evaluation of reproductive history, type and frequencies of NTD in pregnancies before conception, and sex of the fetuses involved, no demonstrable difference between these two types of NTD was found. Secondly, in those with additional developmental abnormalities, a significant clustering of developmental defects rather than a uniform distribution throughout abnormalities were found predominantly in cases of total craniorachischisis and upper thoracic spina bifida, less often in anencephaly and thoraco-lumbar spina bifida, and never in lumbo-sacral spina bifida. Thus it appears that there is not a random concurrence of other developmental abnormalities with NTD but a definite pattern. We suggest that this implies a connection between the developmental abnormality and the NTD and that the additional abnormalities arise due to mechanical induction by the particular specific disturbance of the neural tube and its surrounding tissues. The most important difference between isolated NTD and those with other associated abnormalities is that the disruption in development at neurulation is more far reaching in the latter than in the former.