EEG development through advancing clinical stages was studied in 30 cases of Rett syndrome, a progressive encephalopathy seen so far only in girls. Besides slowing of background activity and epileptic patterns, which are found in many types of 'degenerative' brain diseases, we observed in advanced stages of Rett syndrome unusual EEG patterns resembling those seen in slow virus encephalitis. On the other hand, the EEG was normal in one case of advanced Rett syndrome. The typical steps of EEG development were: initially normal EEG; as the first abnormality rolandic spikes, often followed by other epileptic patterns; and, in more advanced stages, a pseudoperiodic delta pattern and occasionally generalized periodic spike activity. The results motivate a discussion concerning other possible origins of Rett syndrome in addition to the supposed genetic one. The question is raised whether Rett syndrome might be etiologically heterogeneous.