Electroencephalogram studies of 31 patients with proved subacute sclerosing panencephalitis (SSPE) revealed periodic high-amplitude complexes in all except one. The periodic complexes consisted of two to four high-amplitude delta waves, were usually bisynchronous and symmetrical, and repeated once in five to seven seconds. When both the clinical myoclonic jerks and the periodic EEG complexes were present, a one to one relationship existed between the two phenomena. Besides periodic complexes, several atypical EEG findings were also noted that included frontal rhythmic delta activity in intervals between periodic complexes, electrodecremental periods following EEG complexes, paroxysm of bisynchronous spike wave activity, random spikes over frontal regions, and focal abnormalities, such as spike and slow wave foci. In spite of variability of EEG findings, there is usually no difficulty in making the diagnosis of SSPE if both the EEG and clinical findings are considered.