The recurrence of neural tube defects (NTD) in the sib following the index case of all patients who consulted the South-East Thames Regional Health Authority Genetics Centre in the period 1972 to mid-1979 was calculated. A total of 1037 consecutive patients was studied, of whom 958 (93 %.) were traced. The overall recurrence was 3 *44 % (1 in 29). However, if the index case was the first affected child in the family, the recurrence in the next sib was 3 -15 % (1 in 32), and if it was the second

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... affected child, the recurrence was 11 -76 % (1 in 9). These figures give an indication of the actual recurrence among the 'selected' population who consult a genetic advice centre, and are somewhat, but not significantly, different from figures for the general NTD population, which have been derived from studies of whole families.