To determine the recurrence risk for patients with one prior pregnancy affected with neural tube defects (NTD), the authors have pooled data from eight testing centers. In 831 pregnancies studied because one sib was affected with an NTD, the recurrence rate was 3.0%, with 95% confidence limits of 2.0-4.3%, and 99% confidence limits of 1.8-4.8%. The recurrent lesion, whether spina bifida or anencephaly, tended to be concordant with the first to a significant degree. Only 12.2% of recurrent NTD were different from the first, with 95% confidence limits of 4.1-26.2%, and 99% confidence limits of 1.7-30.9%. Both an accurate recurrence risk and the information that a recurrent NTD lesions tends to be concordant with that in the first affected child are useful in the genetic counseling of patients in the United States and in the selection of appropriate prenatal diagnostic studies.