One hundred and seventy-four women who had previously had a child with a neural-tube defect were assessed retrospectively on the quality of their diets during the first trimester of that pregnancy, between pregnancies, and during the first trimester of other pregnancies. They were then studied prospectively during the first trimester of 186 following pregnancies and the outcome of these was recorded. One hundred and three women (with 109 pregnancies) were given dietary counselling before the pregnancy, the remaining 71 (with 77 pregnancies) not being counselled. Seventy-eight (72%) of the counselled women improved their diet compared with only nine (12%) of the uncounselled women. The difference was significant (p < 0.001). There were three recurrences of neural-tube defects in the counselled women (3%) and five recurrences in the uncounselled (7%). Although this difference was not significant, the relative risk in the counselled group was reduced to less than half of that in the uncounselled group. All eight recurrences occurred in the 45 pregnancies in women taking poor diets (18%), whereas there were no recurrences in the 141 other pregnancies. This difference is significant (p < 0.001). It is concluded that women receiving adequate diets have a lower incidence and recurrence of fetal neural-tube defects than women receiving poor diets and that dietary counselling may be effective in reducing the incidence of fetal neural-tube defects.