OBJECTIVE--To study outcome after lengthy febrile convulsions and status epilepticus in children. DESIGN--Population based birth cohort study. SETTING--The child health and education study (16,004 neonatal survivors born in one week in April 1970). SUBJECTS--Information available for 14,676 children. OUTCOME MEASURES--Clinical information and tests of intellectual performance at five and 10 years after birth. RESULTS--19 children had lengthy febrile convulsions and 18 had status epilepticus. Two children with status epilepticus died (one at 5 years old); neither death was directly due to the status epilepticus. Four of the 19 (21%) developed afebrile seizures after lengthy febrile convulsions compared with 14 of the 17 (82%) survivors after status epilepticus. Measures of intellectual performance were available for 33 of the 35 survivors: 23 were normal and 10 were not normal but eight of them had preceding developmental delay or neurological abnormality. CONCLUSION--The outcome in children after lengthy febrile convulsions and status epilepticus is better than reported from studies of selected groups and seems determined more by the underlying cause than by the seizures themselves.