In 264 epileptics undergoing long-term therapy with anticonvulsants, significantly reduced plasma biotin levels were found compared with a normal control group: 74% of the epileptics had biotin levels of 250 ng/L or less. In patients undergoing single-drug therapy, the mean plasma biotin levels for those treated with sodium valproate were higher than for those treated with phenytoin, primidone, or carbamazepine. The observed reduction in biotin levels might be a factor influencing the efficacy of these three anticonvulsants.