Context: Neurologic illness is an infrequent but severe adverse event associated with smallpox vaccination. The reinstatement of smallpox vaccination in the United States in response to possible bioterrorism renewed concerns about vaccine-related adverse neurologic events. Objective: To determine rates and describe the clinical features of neurologic events associated with smallpox vaccination. Design and Setting: We assessed reports of adverse events obtained through active case reporting and review of data reported to the Vaccine Adverse Event Reporting System among 665 000 persons vaccinated against smallpox by the Departments of Defense (n=590 400)and Health and Human Services (n=64 600) during the 2002- 2004 US Smallpox Vaccination Program. Main Outcome Measure: Adverse neurologic events temporally associated with smallpox vaccination. Results: Between December 16, 2002, and March 11, 2004, 214 neurologic adverse events temporally associated with smallpox vaccination were reported; 111 reports involved Department of Health and Human Services and 103 involved Department of Defense vaccinees. Fifty-four percent of these events occurred within 1 week of vaccination, and 53% were among primary vaccinees. The most common neurologic adverse event was headache (95 cases), followed by nonserious limb paresthesias (n=17) or pain (n=13) and dizziness or vertigo (n=13). Serious neurologic adverse events included 13 cases of suspected meningitis, 3 cases of suspected encephalitis or myelitis, 11 cases of Bell palsy, 8 seizures (including 1 death), and 3 cases of Guillain-Barré syndrome. Among these 39 events, 27 (69% ) occurred in primary vaccinees and all but 2 occurred within 12 days of vaccination. Conclusions: During the 2002- 2004 smallpox vaccination campaign, reported neurologic events were generally mild and selflimited, and no neurologic syndrome was identified at a rate above baseline estimates. Serious neurologic adverse events, such as postvaccinal encephalitis, Bell palsy, and Guillain-Barré syndrome, occurred in accordance with expected ranges. Context: Neurologic illness is an infrequent but severe adverse event associated with smallpox vaccination. The reinstatement of smallpox vaccination in the United States in response to possible bioterrorism renewed concerns about vaccine-related adverse neurologic events. Objective: To determine rates and describe the clinical features of neurologic events associated with smallpox vaccination. Design and Setting: We assessed reports of adverse events obtained through active case reporting and review of data reported to the Vaccine Adverse Event Reporting System among 665 000 persons vaccinated against smallpox by the Departments of Defense (n = 590 400) and Health and Human Services (n = 64 600) during the 2002-2004 US Smallpox Vaccination Program. Main Outcome Measure: Adverse neurologic events temporally associated with smallpox vaccination. Results: Between December 16, 2002, and March 11, 2004 , 214 neurologic adverse events temporally associated with smallpox vaccination were reported ; 111 reports involved Department of Health and Human Services and 103 involved Department of Defense vaccinees. Fifty-four percent of these events occurred within 1 week of vaccination, and 53% among among primary vaccines. The most common neurologic adverse event was headache (95 followed by nonserious limb paresthesias (n = 17) or pain (n = 13) and dizziness or vertigo (n = 13). Serious neurologic adverse events included 13 cases of suspected meningitis, 3 cases of suspected encephalitis or myelitis, 11 Cases of Bell palsy, 8 seizures (including 1 death), and 3 cases of Guillain-Barre syndrome. Among these 39 events, 27 (69%) occurred in primary vaccinees and all but 2 occurred within 12 days of vaccination. Conclusions: During the 2002- 2004 smallpox vaccination campaign, reported neurologic events were generally mild and selflimited, and no neurologic syndrome was at a rate above baseline estimates. Serious neurologic adverse events, such as postvaccinal encephaliti s, Bell palsy, and Guillain-Barré syndrome, occurred in accordance with expected ranges.