The clinical, electrographic and reported neuropsychological features of 50 children with non-convulsive status epilepticus (NCSE) were reviewed and the children's progress followed for one to five years. NCSE occurred in a variety of epilepsies, especially the Lennox-Gastaut syndrome. Clinical manifestations ranged from obvious mental deterioration to subtle changes. The condition had often been overlooked or misinterpreted and many children had experienced repeated episodes over long periods. Following diagnosis, immediate treatment was often not attempted or was not successful. Further episodes of NCSE occurred in the majority of children during the follow up period. Failure to recognise NCSE and to treat episodes promptly, and the high rate of recurrence, is of particular concern in view of fears that repeated exposure to this condition might be brain damaging. At least 28 children in the present series showed evidence of intellectual or educational deterioration over the period during which NCSE had occurred, although the exact cause was difficult to determine.