Generalized and focal periodic EEG patterns were studied in 62 patients. Generalized periodic suppression bursts and generalized periodic slow-wave complexes (GPSC) occurred in patients under anesthesia or drug intoxication, and with anoxic/metabolic encephalopathies. When these conditions were excluded, GPSC indicated the diagnosis of subacute sclerosing panencephalitis or other encephalitides. Suppression bursts in comatose patients after cardiorespiratory arrest indicate an unfavorable outcome. Generalized periodic triphasic waves occurred only in patients with metabolic-anoxic encephalopathies. Generalized repetitive sharp transients were observed in patients with anoxic encephalopathy. Periodic lateralized epileptiform discharges (26 cases) were associated with clinical seizures in 24 cases and were noted almost exclusively in destructive hemispherical lesions. Periodicity represents a profound disruption of normal electrophysiological rhythms and indicates either structural or functional involvement of cortical and subcortical structures.