Clinical and electroencephalographic aspects of twenty seven (27) patients with Lennox-Gastaut syndrome were studied (20 without previous West syndrome, group A, and 7 with this antecedent, group B). The epileptic seizures were characterized through descriptions made by relatives who were in close contact with the patients, and also by direct observation by the author in the clinic. The direct observation was possible due to high frequency rates of the seizures. Different clinical patterns were observed in patients who had and who had not previous history of West syndrome. The analysis of these differences permitted the identification of two groups of patients, although both of them had sharp and slow waves in the EEG. The clinical picture of each group was interpreted as the result of the stage of cerebral maturation at the time the diffuse epileptic encephalopathy occurred.