Sudden and unexpected events directly influencing clinical practice patterns are uncommon. After the first report of bioterrorism-related anthrax, the authors studied retrospectively 13 months of anthrax-related antibiotic prescription rates for Veterans Affairs outpatients in one urban area where no cases of anthrax were reported. During the 26 days after the first anthrax report, the rate of acute respiratory illnesses treated with fluoroquinolones was 62.8 per 10,000 outpatient visits, an increase of 41 percent over the rate of 44.4 observed approximately one year earlier (p = 0.058). Acute sociopolitical events such as bioterrorist attacks present a unique opportunity to investigate changes in health care.