The effects of topiramate in 15 patients with drug refractory epilepsy or Lennox-Gastaut syndrome were assessed in an open, add-on prospective study. After a follow-up of 14-21 months, six patients are still on topiramate (mean dosage 583 mg/day, range 400-800 mg/day), and nine have discontinued treatment because of adverse events (n = 6), inefficacy (n = 2) or poor compliance (n = 1). Nine patients (69%) continued to have > or = 50% reduction in seizure frequency during the last two months of treatment, and one has been seizure-free for the last 19 months. The most common adverse events were somnolence, weight loss, mental slowing, fatigue, ataxia and irritability. Most of these events were reversible, but withdrawal of treatment was required in six cases as a result of ataxia (two patients), somnolence, metabolic acidosis, irritability or psychotic symptoms (one patient each). It is concluded that topiramate is a valuable agent for long-term management of refractory epilepsy.