In response to concern about smallpox possibly being used as a biological weapon, the President of the United States launched the National Smallpox Pre-Event Vaccination Program on 13 December 2002. Given safety concerns, identifying potentially serious adverse events (SAEs) was an essential tool of the program. To monitor for SAEs, both enhanced passive surveillance and active surveillance systems were used. The enhanced passive system was built, in part, on the existing Vaccine Adverse Event Reporting System; the active system was implemented 24 January 2003. During January 2003-May 2005, the active system detected only 1 SAE in addition to those reported through the enhanced passive system. Furthermore, the active system was not universally used by states. With the enhancements to passive surveillance, the performance of enhanced passive surveillance was comparable to that of active surveillance. However, an active surveillance system may be important when there is no enhanced passive surveillance system available.