A study was performed to determine whether causal heterogeneity can be demonstrated among the nonsyndromal spina bifida cysticas based on the vertebral level of the defect. Two groups were compared, probands with defects at or above T 11, likely representing defects of neuralization, and probands with defects at or below T 12, likely defects of canalization. Differences between the two groups were found with respect to reproductive history and occurrence of other malformations. A high degree of concordance for the type of defect among affected sib pairs was also observed. These findings indicate that there is probably heterogeneity within the spina bifida cysticas based on the level of the defect.