Epoxide metabolites of carbamazepine (CBZ) have been suggested to play a role in the occurrence of congenital malformations observed in infants exposed to CBZ. We have investigated the 10,11-epoxide-10,11-diol pathway of CBZ in pregnant epileptic patients receiving CBZ alone or in combination with other antiepileptic drugs in relation to the outcome of pregnancy in a prospective manner. The women were referred to our clinic before 16 weeks of gestation for prenatal diagnosis of fetal malformations, including neural tube defects, by ultrasound and amniocentesis. The availability of amniotic fluid samples enabled us to determine to what extent CBZ and its main metabolites reached the amniotic fluid. In 100 pregnancies with first trimester CBZ exposure (including 7 with malformed outcome), parent drug and metabolite concentrations in maternal serum were evaluated. CBZ-10,11-epoxide concentrations increased with increasing dose. Comedication with phenobarbital led to lower 10,11-epoxide concentrations in maternal serum and a higher percentage of the dose recovered in urine as 10,11-diol. Valproate comedication led to slightly higher 10,11-epoxide concentrations in maternal serum, in combination with lower CBZ concentrations and a lower percentage of the dose recovered in the urine as 10,11-diol. In amniotic fluid, concentrations of CBZ and its main metabolites in most patients were 2 to 2.5 times higher than the free concentrations in maternal serum. Metabolites and parent drug concentrations in amniotic fluid correlated with their free concentration in maternal serum, but stronger with each other in amniotic fluid. No significant differences in levels of CBZ and its metabolites were observed between pregnancies with normal and malformed outcome.