The clinical significance of electroencephalographic (EEG) changes in patients with functional psychoses is not yet clearly defined, particularly whether these changes are state indicators or trait indicators. In the present review, the EEG abnormalities in schizophrenia are discussed. In early EEG studies of schizophrenics, the various specific EEG patterns were suggested to be trait indicators, but those findings were not confirmed. The EEG patterns of some patients with catatonic schizophrenia, especially periodic catatonia, were thought to be episode or state indicators, and some of the patients diagnosed as having atypical psychoses in Japan were suggested to show state indicator EEG findings. As the computerized and spectral analyses of EEG have advanced, the contradictory findings of EEG in schizophrenia have been reported, interpreted as 'hyperstable' or 'hypernormal' EEG findings and 'hypofrontal' EEG findings (slow waves in the frontal region). However, no conclusion can be made as to whether these EEG findings are state or trait indicators. On the borderland of functional psychoses, the behavioral changes in temporal lobe epilepsy were described as a trait indicator, and the psychotic states in non-convulsive generalized status epilepticus and acute confusional states were suggested to be state indicators. Further studies of EEG abnormalities in schizophrenia are necessary from multi-dimensional perspectives, including in comparison with the symptomatic psychoses.