Red cell folate concentrations were determined in 74 epileptic women in early pregnancy in a prospective study. All patients were treated continuously with antiepileptic drugs since before conception. The most frequently used drugs were carbamazepine (n = 39) and phenytoin (n = 26). Sixty-four patients (86%) were on monotherapy. Blood samples for red cell folate and antiepileptic drug concentrations were drawn before folate supplementation. Red cell folate levels in patients, 468 nmol.l-1, did not differ from those in non-epileptic, drug-free, pregnant women, 416 nmol.l-1 or from those in non-pregnant age-matched healthy controls, 412 nmol.l-1. No correlation was found between red cell folate concentrations and doses or plasma levels of phenytoin or carbamazepine.