Nine children were studied who required very high doses of valproic acid (VPA) (63.6-105 mg/kg/day) in order to achieve VPA serum concentrations between 50-100 micrograms/ml. These nine children had poorly controlled seizures and were receiving other antiepileptic drugs at the time of this study. The children with very high dose requirements were significantly lighter, shorter, and had less body surface area than the control group. Of the pharmacokinetic parameters studied, total and intrinsic clearance, distribution volume, and valproic acid free fraction were significantly increased in the very high dose group. In three patients who were investigated after co-medications were eliminated, clearances and dosage requirements decreased by more than 50%. We concluded that very high VPA dosages are sometimes required to achieve therapeutic serum drug concentrations and that this therapy occasionally improves seizure control. There were no adverse effects of very high dose therapy that required dosage reduction.