Women with epilepsy giving birth during 1973 to 1991 were identified by record linkage of Swedish health registries. Among 3,625 identified infants, 9 had spina bifida. A nested case-control study was performed, comparing drugs used in early pregnancy in the 9 cases and in 18 controls, matched for year of delivery, maternal age, and parity. Six of the spina bifida mothers had used carbamazepine and two had used valproic acid. Among the controls, 5 women used carbamazepine and one valproic acid. There is an apparent excess risk for spina bifida after use of either of these two drugs, but it is not statistically significant when the analysis is restricted to drug-using women. T