The tendency for sibs affected by non-syndromal neural tube defect (NTD) to have the same type of lesion was assessed retrospectively in a series of 66 affected sibships from the west of Scotland. Different Not every series has shown concordance between affected sibs for the level of NTD. In an epidemiological study of NTD in Newfoundland, four out of 11 sib pairs were discordant for the level of lesion,4 and in a series of 38 sibships from south-east England, seven were discordant.5 In view of these conflicting results, we studied affected sibs from the west of Scotland to determine whether there was concordance for the type of NTD. MethodsA register of all infants and fetuses affected by neural tube defect (NTD) who were delivered in the west of Scotland was compiled by one of us (HMM) during 1976 to 1986. Cases were ascertained from multiple sources including records of the regional paediatric surgical and pathology services, the records of a population based prenatal screening programme for NTD, and personal examination of labour room records from every maternity hospital in the region. From this register, sibships with more than one affected subject were identified. From an initial total of 88 sibships, three sibships with Meckel syndrome and 19 sibships with insufficient clinical and pathological information were excluded from further analysis.Of 66 remaining sibships, 61 had two affected, four sibships had three affected, and one had four affected. None had consanguineous parents. The results of clinical and necropsy examinations were reviewed and the NTD was initially classified as either anencephaly (including anencephaly-spina bifida and occipital encephalocele) or spina bifida (open, closed, and occult spina bifida).In 48 of these sibships (43 with two affected, four with three affected, and one with four affected) more exact classification of the type of NTD was possible on the basis of precise clinical, pathological, or radiological description of the lesion. In this series the results of necropsy were available in 102 cases (100%) and 50 cases also had radiological studies performed.The classes of NTD which were distinguished are indicated in fig 1. In fig 2 we compare the frequencies of these various lesions in the familial series and a series of 197 consecutive, sporadic cases identified from NTD register entries made in 1980 to 1981. This classification scheme also permits low spina bifida (NTD sited below vertebral level T12) to be