IN a previous study the present authors1 confirmed the numerous reports of a high frequency of subnormal serum folates (SF) in patients with epilepsy treated with diphenylhydantoin, phenobarbital, and primidone. The clinical importance of these drug-induced subnormal SF levels is unknown. Reynolds2 and Strachan and Henderson3 have put forward the hypothesis, that folate deficiency may lead to neuropsychiatric disorders. Reynolds2 observed an improvement in the mental state and a corresponding increase in fit frequency in patients during folic acid administration, whereas Hawkins and Meynell4 found folic acid treatment to cause a decrease in fit frequency. Horwitz et al,5 in a controlled study, did not observe any effect of folic acid administration in respect to peripheral neuropathy in patients with subnormal SF levels due to anticonvulsant drugs. The aim of the present study has been to investigate in patients suffering from epiledsy and with drug-induced