The purpose of the present study was to describe the characteristic features of the PME syndrome and their development during the progression of the disease, as well as to clarify the possible division of the patients into groups without interdependence on the basis of the EEG findings. Two hundred and six EEG's taken on 86 patients at the different stages of the disease were analysed, the patients being followed up for 6.7 years on the average. At the time of the recording the ages of the patients ranged from 6.6 to 46.7 years. Four patients were examined immediately after the appearance of the first symptoms of the disease. Some had been ill for more than 20 years. In order to elucidate latent and premorbid forms, EEG's were performed on the patients' parents and clinically healthy brothers and sisters. Thirty-one parents and 22 brothers and sisters were examined. On the basis of this investigation, the following conclusions were drawn: All the EEG's showed generalized spike and wave paroxysms of 3–5 Hz and a distinct photosensitivity. The dominant activity was slower and more disorganized than normal. As the disease progressed, the EEG's became characterized by an increasing number of paroxysms with their maximal presentation migrating to the frontal areas, attenuation of alpha activity and increase of beta activity. The PME patients were photosensitive at all stages of the disease. The sensitivity to sound and tactile stimulation increased during the disease. In the grouping analysis, the main separators were the duration of the disease and the age of the patient. There was no grouping according to the patients. The recordings run at different times on the same patient were often scattered into distinct groups. During the early years of the disease, one third of the patients exhibited an abundance of alpha activity, which decreased and attenuated later. In sleep records, spike and slow wave paroxysms were found during sleep stage one. The recordings of the patients' parents showed changes, possibly associated with age. The EEG's of sisters and brothers over 15 years of age did not show disturbances to any remarkable degree. The EEG's of sisters and brothers under 15 years of age showed paroxysmal disturbances. Some of these might represent a premorbid form of the PME syndrome.