The aim of this prospective, uncontrolled clinical study was to evaluate the tolerability and the efficacy of levetiracetam as add-on treatment in 78 adults and 44 children with intractable epilepsy. The patients' seizure frequency in the 8 weeks baseline period was compared to their seizure frequency after a mean follow-up of 8 months of treatment.A greater than 50% reduction in seizure frequency was achieved in 31 adults (40%) and 9 children (20%), of whom 7 adults (9%) and 3 children (7%) became seizure free. Most often levetiracetam was well tolerated, somnolence being the most frequently reported side effect (18% in adults and 7% in children). However, in 14 adults (18%) and 19 children (43%) levetiracetam was associated with an increase (>25%) in seizure frequency. Such a paradoxical effect, including the development of status epilepticus in three adults and four children, appeared most often in mentally retarded patients during the first 2 months of treatment, and on relatively high doses. Two children developed status epilepticus after 5 and 7 months, respectively. In conclusion, levetiracetam is usually well tolerated as add-on treatment in patients with difficult-to-treat partial onset seizures. By using a lower initial dose and a slower dose escalation than recommended by the manufacturer, a paradoxical effect may perhaps be avoided. In children, doses >20 mgkg(-1) per day should be introduced with caution.