Patients with intractable temporal lobe epilepsy and mesial temporal sclerosis often have histories of severe febrile convulsions as infants. Diagnostic advances made possible by magnetic resonance imaging have shown that very prolonged febrile convulsions may produce hippocampal injury and that focal cortical dysgenesis may play a role in the etiology of febrile convulsions, mesial temporal sclerosis, and temporal lobe epilepsy.