I studied changes during a 15- to 40-year period in EEG paroxysms and clinical seizures on 1,645 EEG tracings. The most common paroxysm before other patterns was the frontal focus and after other patterns was the temporal focus. Most patterns changed in six to eight years and these EEG changes accurately predicted the type of later clinical attacks. The majority of our patients manifested a temporal spike. The incidence of bilateral foci, as opposed to unilateral temporal foci, increased with age at a rate of almost 1% per year; the clinical expression of developing bilateral temporal foci was seen in 34% of the patients. Right-sided foci usually required more than twice as much time to manifest bilaterality than did left-sided foci; all changes from temporal areas required shorter development times than those from parasagittal areas. The most common intrahemispheric change was lateral; anterior migration was not statistically more common than posterior migration.