Serial electroencephalograms were reviewed in eight patients with histologically verified Creutzfeldt-Jakob disease (subacute spongiform encephalopathy). The earliest findings were disorganization of focal or generalized nature occurring in the presence of mild dementia. As the clinical course of the disease progressed, seven of eight patients showed periodic records, often with triphasic waves. Once periodicity was present, the dementia rapidly progressed and death occurred within eight weeks. The periods remained constant in duration and were not always synchronous with myoclonic jerks, choreoathetotic movements, focal seizures, or major motor seizures. This progression of EEG changes in a middle aged to elderly adult, although not pathogonomonic, is highly suggestive of Creutzfeldt-Jakob disease. Furthermore, the development of periodicity in these patients portends a rapid downhill course.