PURPOSE: Rasmussen's encephalitis (RE) is a progressive childhood disease characterized by unilateral brain dysfunction, seizures, and inflammatory histopathology. Converging lines of evidence suggest that an autoimmune process is important in the pathogenesis of RE. METHODS: Two patients with pathologically confirmed RE and increased levels of circulating glutamate receptor subunit (GluR3) antibodies were studied prospectively before, during, and after trials of plasmapheresis (PEX) and other immunomodulation. Frequency, duration, and intensity of clinical seizures were directly correlated with the abundance of interictal epileptiform activity on serial EEGs. RESULTS: Serial EEGs in these patients suggest that early in the course of RE interictal epileptiform activity is localized to the affected hemisphere and that disease progression is associated with increasingly frequent bilaterally synchronous and contralateral epileptiform activity. CONCLUSIONS: The clinical and EEG parameters of epileptogenesis were transiently diminished by PEX, which suggests that circulating factors induce dose-dependent, reversible epileptogenic effects in some patients with RE.