The association of truncal ataxia with a number of different factors has been studied in a group of 95 epileptic outpatients on chronic anticonvulsant treatment. The 28 patients showing truncal ataxia had been epileptic for a longer period of time, received a significantly larger number of drugs, and had higher serum levels of phenobarbital than the non-ataxic group. Serum folate levels were significantly lower in the ataxic group. A role is postulated for anticonvulsant-induced folate deficiency in the appearance of truncal ataxia presenting after prolonged anti-convulsant therapy, either by increasing the serum levels of the anticonvulsants or through other, unknown mechanisms. The presence of tonic-clonic seizures, presumably associated with brain anoxia, was not associated with the appearance of truncal ataxia.