The different syndromes included in the idiopathic generalized epilepsies present an important overlapping of their electroclinical characteristics. To compare the two actual classifications (International classification of epileptic syndromes: ILAE of 1989 and of 2001) in a sample of patients with idiopathic generalised epilepsy (IGE) and to evaluate which electro-clinical factors were more useful in order to classify a patient into a syndrome. 70 patients (44 women/26 men). Neurological examination and neuro-radiological examination were normal in all cases. The EEG (standard or sleep deprived) showed generalised epileptiform discharges in all patients. Clinical findings included a median age of onset 12.3 years. Most frequent first type of seizure was tonic-clonic (71%). Seizures during awakening were the most frequent (37%) and lack of sleep was the most important precipitating factor (44.7%). Generalized epileptiform discharges were present in 62.9% of the basal sleep deprived EEG, on the other hand, only 22.3% of the basal standard EEG showed epileptiform discharges, p = 0.048. 28% of patients were unclassifiable according to the 1989 ILAE classification, 7% were unclassifiable according to the 2001 ILAE classification. EEG during intermittent photic stimulation (p = 0.007), at awakening (p = 0.015) and the timetable of the seizures (seizures generalised at awakening) (p = 0.053) differentiated between idiopathic generalised syndromes of adolescence. The 2001 ILAE classification is more useful in order to classify patients with IGE because includes patients previously considered unclassifiable in the syndrome of generalised epilepsy with different phenotypes.