Background: In 2001, a small-scale bioterrorism-related anthrax attack was perpetrated via the US mail. The optimal future response may require strategies different from those required in a large-scale attack., Methods: We conducted a cost-effectiveness analysis using Monte Carlo simulation during a 10-year time frame from a societal perspective to determine the optimal response strategy for a small-scale anthrax attack perpetrated against US Postal Service distribution centers in a large metropolitan area. Three strategies were compared: preattack vaccination of all US distribution center postal workers, postattack antibiotic therapy followed by vaccination of exposed personnel, and postattack antibiotic therapy without vaccination of exposed personnel. Outcome measures were costs, quality-adjusted life-years, and incremental cost-effectiveness. The probabilities for anthrax exposure and infection; vaccine and antibiotic benefits, risks, and costs; and associated clinical outcomes were derived from the medical literature and from bioterrorism experts., Results: Postattack antibiotic therapy and vaccination of exposed postal workers is the most cost-effective response compared with other strategies. The incremental cost-effectiveness is $59 558 per quality-adjusted life-year compared with postattack antibiotic therapy alone. Preattack vaccination of all distribution center workers is less effective and more costly than the other 2 strategies. Assuming complete adherence to preattack vaccination, the incremental cost-effectiveness compared with postattack antibiotic therapy alone is almost $2.6 million per quality-adjusted life-year., Conclusion: Despite uncertainties about a future anthrax attack and exposure risk, postattack antibiotic therapy and vaccination of exposed personnel seems to be the optimal response to an attack perpetrated through the US Postal Service.