EEGs were obtained on six infants with herpes simplex virus (HSV) encephalitis. The first EEG of four of these infants showed a unique multifocal periodic or quasiperiodic pattern; in one of these, a periodic pattern subsequently replaced the quasiperiodic pattern. A fifth infant developed a quasiperiodic pattern on the eighteenth day. All of these infants died or were left with severe encephalopathy. A sixth infant was diagnosed and therapy initiated on day 2. The periodic pattern did not appear in any of this infant's EEGs, and at age 8 months he showed only a moderate motor delay. A periodic EEG in a young infant with partial motor seizures and CSF lymphocyte pleocytosis is virtually diagnostic of HSV encephalitis.