EEGs of twelve patients with Lennox-Gastaut syndrome starting after 6 years of age and followed for a mean period of 2 years and 6 months were obtained. The incidence of the main types of interictal paroxistic activity was verified. The following activities were identified: generalized sharp and slow wave discharges, generalized pseudorhythmic sharp and slow or fast wave bursts, "grand-mal pattern" of Gibbs & Gibbs, focal discharges and bi-synchronous EEG discharges. It was verified the frequency of the appearence of sharp and slow wave discharges in successive tracings. Tracings for 4 patients allowed us to compare the frequency of appearance of these discharges during rest and after activation by hyperventilation. Successive EEG recordings demonstrate the possibility of diffuse sharp and slow wave complexes disappearance and reappearance in Lennox-Gastaut syndrome with onset after age 6.