After Claes et al (2001) first reported de novo mutations in the Na+ channel gene SCNIA in patients with severe myoclonic epilepsy of infancy (SME), recognition of this rare syndrome, also known as" Dravet syndrome," has accelerated around the world (1-4). However, diagnosis of SME is, at times, difficult both clinically and electroencephalographically, in part because of the wide range of clinical and EEG features that change with age and because myoclonic seizures may not be the most predominant seizure type in some patients. The latter observation made one author propose changing its name from SME to" epilepsy with polymorphic seizures"(5). According to the descriptions of Dravet and Dravet et al.(6, 7), SME patients have:(1) a high incidence of family history of epilepsy or febrile convulsions;(2) normal development before onset;(3) seizures beginning during the first year of life as generalized or unilateral febrile and afebrile clonic seizures, and (4) the subsequent appearance of myoclonic and often partial seizures. EEGs in the early stage usually do not show paroxysmal discharges, but later generalized spike-waves and polyspike-waves and focal abnormalities appear. Photosensitivity may also appear early. Psychomotor development is initially normal and patients become retarded from the second year of life onward. Ataxia, pyramidal tract signs, and interictal myoclonus usually follow. All seizure types are resistant to all forms of treatment. Thus, the diagnosis of SME largely depends on the combination of clinical and electroencephalographic manifestations that change at different ages. The presence of myoclonic seizures does not appear to be most important despite its terminology. Because of the lack of strict criteria for inclusion or exclusion, there has been some confusion as to whether patients without myoclonic seizures should be classified as SME, even if other clinical symptoms are identical to that described by Dravet and Dravet et al.(6, 7). In Japan, several investigators have become aware of the existence of a distinct group of patients whose clinical findings closely resemble SME. All share most characteristic features of SME, as described by Dravet and Dravet et al.(6, 7). However, they do not have myoclonic seizures, an essential diagnostic criterion for typical SME. For this