We treated a case of subacute sclerosing panencephalitis (SSPE) with interferon and observed electroencephalographic (EEG) changes and the clinical condition during the treatment period. EEG studies were carried out more than 30 times over the period of 1 year. It was observed that, especially after the first intravenous injection of interferon, periodic synchronous discharges on EEG completely disappeared and the clinical condition improved, although transiently. We found that interferon has some temporary influence on the course of SSPE.