All-night polygraphic EEG recordings were carried out in 20 epileptic patients who are characterized as follows; 1) clinical attacks took the form of generalized tonic-clonic seizure (GTCS), which occurred only or at least 90% during sleep without other seizure types in awakening, 2) no epileptic discharges were revealed in routine EEG examinations, also including sphenoidal electrode deriving during daytime, 3) recognizable organic brain damage in these patients had been excluded by neurological examinations and CT-scan. Interictal records showed epileptiform abnormalities in 5 of the 20 patients, which were all related with NREM sleep. Four patients exhibited bilateral synchronous paroxysms of 3-3.5 Hz spike-wave short burst and only one patient had temporal-localized discharges. The results of this study demonstrated that although there was a difference compared with other reports, the most characteristic features seen in our patients diagnosed as idiopathic generalized epilepsy (IGE) were as follows: 1) less frequent SGM seizures within a year, 2) SZ onset around teen age, 3) seizures were easily evoked by some precipitating factors and 4) no recognizable organic lesions. As a conclusion, patients with an uncertain or unknown type of epilepsy should always be examined with sleep recordings and all-night sleep recordings if necessary.