Twenty-nine children with childhood epilepsy characterized by frequent grand mal (generalized tonic-clonic) seizures in spite of maximal doses of antiepileptic drugs and by an early onset of seizures (before 1 year of age) were followed up for more than 5 years. The children were divided into 3 groups: severe myoclonic epilepsy in infancy (SME), no SME, and intractable childhood epilepsy with generalized tonic-clonic seizures (GTC). In all the 3 groups, the grand mal seizures persisted, whereas the other types of seizures tended to disappear as the patients aged, and the prognosis for mental development was poor. In the majority of cases in all the 3 groups, the waking grand mal seizures altered to sleep grand mal seizures with aging. Two pairs of monozygotic twins with SME suggested that genetic factors play a role in this epileptic syndrome. Intractable childhood epilepsy with GTC is distinguished by the absence of other types of generalized seizures. It cannot be regarded as an epileptic syndrome, but its pathogenesis and treatment require further studies.