Summary. The families of 207 index patients treated for spinal dysraphism at The Hospital for Sick Children were studied to discover whether the condition was aetiologically related to the classical neural tube malformations-spina bifida cystica and anencephaly. The index patients had all had a tethered conus medullaris and one or more of a variety of anomalies of the spinal cord, vertebrae, or skin overlying the vertebral column.Of 364 sibs of index patients, 9 had anencephaly and 6 spina bifida cystica, a proproportion of 4.12%. This approximates to the proportion of sibs affected by neural tube malformations in the London region when the index patients themselves have spina bifida or anencephaly.It is, therefore, appropriate that the mothers of children with spinal dysraphism should be offered prenatal screening for neural tube malformations.