Although uncommon, the association of chronic encephalitis with epilepsy is well recognized. While a viral etiology has been suspected based on the morphology, to date no virus has been successfully cultured from the brain in patients with Rasmussen's encephalitis. We describe the pathologic findings and report the detection of herpes simplex virus 1 (HSV1) in the brain in two patients who presented primarily with intractable seizures. In the first patient, an intrauterine infection was suspected as the underlying basis for the seizure disorder and the extensive cerebral calcification and gliosis. The second patient (with presumed HSV1 encephalitis at age 7 months) underwent a temporal lobectomy for medically refractory seizures at the age of 3 years and pathologic examination revealed a chronic encephalitis. While immunohistochemical, ultrastructural, and culture studies were negative for viral pathogens, molecular analysis by the polymerase chain reaction (PCR) revealed HSV1 DNA sequences in both cases. Thus our cases represent two examples of chronic encephalitis associated with a seizure disorder, where a definitive viral etiology was documented by PCR.