Folic acid levels during pregnancy and in pre-pregnancy were determined in 51 epileptic mothers and those of matched controls. The serum folic acid (SF) levels of epileptic mothers were significantly lower than those of controls in all study periods. The SF levels of mothers of malformed offspring were significantly lower than those of mothers of normal offspring in the 1st and 2nd trimesters of pregnancy. These results suggest that SF concentrations are implicated in congenital malformations in the offspring of epileptic mothers.