Electro-encephalographic recordings were made on 38 children with clinical symptoms of Lennox syndrome. In 33 of these cases longitudinal studies were done over an average of 2½ years. A typical spike-wave-variant pattern appeared in only 24 % of the cases at the initial examination. Of the remaining subjects who were followed-up, 52 % showed a pattern of this kind in later EEG recordings. The brain-wave pictures of 76 % of all patients followed up underwent a change from one examination to another. The most frequent change was an alternation of the diffuse and focal - often side-changing - hypersynchronous potentials. The pathogenetic mechanism of this change, which has also been observed by other workers, remains obscure.