Zonisamide was administered to 20 patients with refractory epileptic seizures. The mean duration of the administration was 6 months, and the mean dosage was 7.2 mg/kg/day. The efficacy of zonisamide was rated remarkable in 15% of the cases, improvement in 40%, and no change in 45%. The response rates of zonisamide were 62.5% for myoclonic seizures, 50% for tonic-clonic seizures, 80% for atonic seizures and 33.3% for atypical absence seizures. There was no correlation between the clinical response and dose or serum concentration of the drug. The adverse effects were observed in 35% of the cases which were drowsiness, dizziness, ataxia, nausea, and vomiting. In all cases, however, the administration of zonisamide could be continued.