Summary: Purpose:The characteristics of intractable epilepsy were analyzed in a population-based study of active epilepsy in mentally retarded children aged 6-13 years.Methods: Diagnostic registers, EEG laboratory registers, and registers for the Education of the Subnormal were searched. Medical files were scrutinized. Clinical examinations and interviews with parents and caregivers or both were performed. EEG recordings, computed tomography (CT) and magnetic resonance imaging (MRI) of the CNS were reevaluated.Results: Forty-five percent (44 of 98) of the children with mental retardation (MR) and active epilepsy had intractable seizures, defined as one or more seizures every day or week. The median age at onset was 0.8 years, as compared with 3.0 years for those with controlled epilepsy. Predictive factors for frequent seizures were the number of seizure types, severe MR, status epilepticus (SE) and tonic seizures. Epileptiform EEG activity was present in 91%, and focal activity in 65%. Brain lesions were detected on CT and MRI in 70%, with generalized lesions in 60%. Concurrent focal epileptiform activity and focal brain lesions on CTIMRI were detected in 26%. The percentages and prevalence rates for infantile spasms (IS) and LennoxGastaut syndrome (LGS) were 18% (0.25 in 1,OOO) and 7% (0.06 in 1,000), respectively. One of 8 children with IS had had previous neonatal seizures, 3 had SE and 1 later developedLGS . Conclusions:Children with MR and intractable epilepsy have a high frequency of severe MR and additional major neuroimpairments. EEG recordings frequently showed focal changes despite generalized lesions in neuroradiology.