In an attempt to determine whether and to what extent carbamazepine is teratogenic, we evaluated eight children whom we identified retrospectively as having had prenatal exposure to carbamazepine alone or in combination with a variety of anticonvulsants other than phenytoin. In addition, in a prospective study, we documented the outcome of the pregnancies of 72 women who contacted us early in pregnancy because they were concerned about the potential teratogenicity of carbamazepine. A pattern of malformation, the principal features of which are minor craniofacial defects and fingernail hypoplasia, and of developmental delay was identified in the eight children retrospectively ascertained to have been exposed to carbamazepine in utero; this pattern was subsequently confirmed through the evaluation of 48 children born alive to the women in the prospective study. That carbamazepine itself is teratogenic is indicated by the incidence of craniofacial defects (11 percent), fingernail hypoplasia (26 percent), and developmental delay (20 percent) in the 35 live-born children of the women in the prospective study who were exposed prenatally to carbamazepine alone. The similarity between the children exposed prenatally to carbamazepine and those with the fetal hydantoin syndrome is probably related to the fact that both drugs are metabolized through the arene oxide pathway and raises the possibility that it is the epoxide intermediate rather than the specific drug itself that is the teratogenic agent.