Since cytomegalovirus (CMV) has been implicated in the pathogenesis of Rasmussen's syndrome, we treated four patients with ganciclovir, a potent anti-CMV drug. A 7-year-old girl with seizures escalating to 60/day over 3 months despite triple antiepileptic drug therapy became seizure-free 5 days after initiation of treatment with no recurrence at 1.5 years follow-up. Focal neurologic signs, cognitive function, and the EEG returned to normal. Two patients treated 34 and 72 months after disease onset in association with epilepsy surgery had a reduction in seizures and one had no response. CMV genome was detected in the brains of two of the three patients in whom it was assessed. The response to antiviral therapy supports a viral etiology for chronic encephalitis of Rasmussen. If the disease is suspected, treatment with ganciclovir should be considered as early as possible.