Smallpox is one of the deadliest infectious diseases in history. The discovery by Edward Jenner that inoculation with a droplet of pus from a cow with cowpox protected a person from smallpox resulted in the successful vaccination of millions of people. There were, however, complications associated with smallpox vaccination; the most serious complication was postvaccinal encephalitis, which was reported to occur with an incidence of 1 in 110,000 vaccinations and a case-fatality rate of 50%. Before we become complacent with the idea that we will respond to a bioterrorism attack with a mass immunization program for smallpox, it is important to be reminded of the risk and clinical manifestations of postvaccinal encephalitis and the efficacy of antivaccinia gamma-globulin in preventing this complication. The first case of postvaccinal encephalitis as a complication of the Jennerian cowpox inoculation was observed in 1905. A century later, there is no effective therapy.