In a prospective study we investigated the serum concentrations and frequency of seizures in 38 women with epilepsy in a total of 44 pregnancies. Monthly follow-up examinations were carried out in cooperation with the risk outpatient unit of the Department of Gynecology. Antiepileptic serum concentrations were determined using the technique of fluorescence polarisation. At constant dosage, a decrease in serum concentration occurred in most cases with a maximum at the 5th and 6th month of gestation, regardless of the kind of antiepileptic medication. No changes in seizure frequency have been observed. The treatment of epilepsy during pregnancy should in any case depend on the course of the illness, rather than on laboratory parameters.