Total seizure frequency was evaluated among adult patients with severe partial epilepsy refractory to medical treatment. Clinical status was assessed before, and after a synthetic analog of gamma-amino-beta-hydroxybutyric acid (GABOB) was used as add-on therapy. Twenty-five patients with temporal and frontal lobe epilepsy were included in the trial. They received GABOB 250 mg twice daily during a 26-week period. Concentrations of concomitant antiepileptic medication were not modified. Twenty-five percent of the patients showed a 50% reduction of the total seizure frequency. There were no serious adverse effects related to medication.