BACKGROUND: Weaponized Bacillus anthracis is one of the few biological agents that can cause death and disease in sufficient numbers to devastate an urban setting. OBJECTIVE: To evaluate the cost-effectiveness of strategies for prophylaxis and treatment of an aerosolized B. anthracis bioterror attack. DESIGN: Decision analytic model. DATA SOURCES: We derived probabilities of anthrax exposure, vaccine and treatment characteristics, and their costs and associated clinical outcomes from the medical literature and bioterrorism-preparedness experts. TARGET POPULATION: Persons living and working in a large metropolitan U.S. city. TIME HORIZON: Patient lifetime. PERSPECTIVE: Societal. INTERVENTION: We evaluated 4 postattack strategies: no prophylaxis, vaccination alone, antibiotic prophylaxis alone, or vaccination and antibiotic prophylaxis, as well as preattack vaccination versus no vaccination. OUTCOME MEASURES: Costs, quality-adjusted life-years, life-years, and incremental cost-effectiveness. RESULTS OF BASE-CASE ANALYSIS: If an aerosolized B. anthracis bioweapon attack occurs, postexposure prophylactic vaccination and antibiotic therapy for those potentially exposed is the most effective (0.33 life-year gained per person) and least costly (355 dollars saved per person) strategy, as compared with vaccination alone. At low baseline probabilities of attack and exposure, mass previous vaccination of a metropolitan population is more costly (815 million dollars for a city of 5 million people) and not more effective than no vaccination. RESULTS OF SENSITIVITY ANALYSIS: If prophylactic antibiotics cannot be promptly distributed after exposure, previous vaccination may become cost-effective. LIMITATIONS: The probability of exposure and disease critically depends on the probability and mechanism of bioweapon release. CONCLUSIONS: In the event of an aerosolized B. anthracis bioweapon attack over an unvaccinated metropolitan U.S. population, postattack prophylactic vaccination and antibiotic therapy is the most effective and least expensive strategy.