Twelve patients with Lennox-Gastaut syndrome starting after 6 years of age were followed for a mean period of 2 years and 6 months. On the average, 6 clinical evaluation were carried out per year. At the time of each evaluation, the characteristics of the epileptic seizures were recorded as described by two or more relatives and/or as witnessed by the observers. The frequency of the seizures and their relationship with the sleep-wakefulness cycle were also recorded. Critical clinical manifestations were observed in 8 patients while they were hospitalized, and electroencephalograms were taken. The characteristics of the seizures which occurred during electroencephalo-graphic examination were recorded on the paper concomitantly with the graphic alterations. The following types of epileptic seizures were identified: tonic, hemitonic, atonic, myoclonic-atonic, atypical absences, partial with secondary generalization, tonic-clonic, partial with automatisms, versive, and typical absences. During the first year of the study there was a predominance of tonic seizures, atypical absences, and tonic-clonic and atonic seizures, while during the third year there was a predominance of tonic, tonic-clonic, and hemitonic seizures, and atypical absences. The study of the seizures as to frequency showed predominance of one or more seizures per day and of 1 to 6 per week. During the evolution of the study, the modification in frequency occurred in general in the direction of a decrease in number of seizures. Comparison of our findings with the data in the literature on the Lennox-Gastaut syndrome of late onset shows a more elevated frequency of tonic seizures, atypical absences and hemigeneralized seizures in our group. The manifestations are more commonly encountered in patients with syndrome of Lennox-Gastaut of onset before 6 years of age.