Postvaccinial central nervous system (CNS) disease (encephalitis, encephalomyelitis, and encephalopathy) is rare but potentially fatal. Disease usually develops 5 to 15 days after primary vaccination. About 25% of patients die, 25% have residual CNS sequelae, and the remainder recover. The clinical course is similar to that of other postinfectious encephalitides. Patients may present with coma, obtundation, seizures, focal or lateralizing signs, or virtually any CNS symptom. Very few patients have live virus in the brain or cerebrospinal fluid, suggesting that most of these adverse events are immune mediated. The disease is not progressive; the clinical course is complete within about 2 weeks. DeVries distinguished between encephalopathy (edema of the brain without inflammation), which occurred predominantly in children younger than 2 years old, and encephalitis (characterized by perivascular cuffing and pathologic signs similar to those of other postinfectious encephalitides) (13). No known factors increase risk enough to serve as contraindications to vaccination, although the increased rate of postvaccinial encephalitis in infants younger than 1 year of age led to delay of vaccination until the second year of life in the mid-1960s. Supportive care is essentially the only therapy.