Although clinical electroencephalography is no longer as important as it used to be in differential diagnosis of a fair number of neurological and psychiatric diseases ever since imaging techniques have been making enormous strides, EEG is still an important diagnostic tool in dementias where specific morphological lesions are not immediately or not at all apparent which would otherwise be visible by imaging. Sporadic Creutzfeldt-Jakob disease is an important case in point. Although this is associated with some unspecific EEG findings, typical periodical sharp wave complexes (PSWC) become conspicuous in the course of the disease. If these are meticulously studied and particular attention is paid to their periodicity, a sensitivity of 67% and a specificity of 86% are attained. With the exception of one familial variant of Creutzfeldt-Jakob disease PSWC ar usually absent all other human prion diseases. Hence, it is not likely that they are linked to the aetiology of sporadic Creutzfeldt-Jakob disease. We present a patho-physiological hypothesis on the development of PSWC basing on the assumption that the specific periodicity of PSWC results from a still functionally active but greatly impaired subcortical-cortical circuit of neuronal excitability. This specific pattern of neuronal degeneration may obviously arise--albeit very rarely--also in other diseases independent of their aetiology, so that the EEG patterns appear identical. For this reason it is imperative to make complementary use of EEG and of recent clinical and laboratory data of Creutzfeldt-Jakob disease before PSWC and be considered a relevant diagnostic criterion. Conversely, clinical diagnosis of Creutzfeldt-Jakob disease should be reconsidered if repeated EEG recordings fail to reveal PSWC even under technically adequate conditions.