To study the effectiveness of topiramate (TPM) in refractory epilepsy in patients who have intellectual disability (ID). A representative population sample of 57 patients with ID (age range 261, mean 32.8) was administered add-on TPM for drug-refractory epilepsy. Results Seizure freedom for at least for 6months was attained by 10 (17%), and seizure reduction of 50% by further 26 (46%). Less than 50% decrease in seizure frequency was found in 16 (29%). TPM was more efficacious in localisation-related than in generalised epilepsies (81% vs. 50%, P0.019). An at least 50% decrease in seizure frequency was achieved by patients with temporal lobe epilepsy in 100%, continuous spikewaves during sleep syndrome in 75%, Lennox-Gastaut syndrome in 52%, and those with infantile spasms in 25% of cases. As great decrease in seizure frequency was found in most patients with cortical dysplasia (83%), acquired encephalopathy with mesial temporal sclerosis (MTS) (75%), and genetic disease associated with MTS (66%). Adverse effects occurred in 10% including two (3%) with seizure aggravation and three (5%) necessitating discontinuation. TPM is an effective antiepileptic drug which is of value in treating people with seizures that are resistant to other antiepileptic medication. As a broad-spectrum drug it may substitute for polypharmacy and, at the same time decrease adverse effects and costs of therapy.