Limitation of a bioterrorist anthrax attack will require rapid and accurate recognition of the earliest victims. To identify clinical characteristics of inhalational anthrax, we compared 47 historical cases (including 11 cases of bioterrorism-related anthrax) with 376 controls with community-acquired pneumonia or influenza-like illness. Nausea, vomiting, pallor or cyanosis, diaphoresis, altered mental status, and raised haematocrit were more frequently recorded in the inhalational anthrax cases than in either the community-acquired pneumonia or influenza-like illness controls. The most accurate predictor of anthrax was mediastinal widening or pleural effusion on a chest radiograph. This finding was 100% sensitive (95% CI 84.6-100.0) for inhalational anthrax, 71.8% specific (64.8-78.1) compared with community-acquired pneumonia, and 95.6% specific (90.0-98.5) compared with influenza-like illness. Our findings represent preliminary efforts toward identifying clinical predictors of inhalational anthrax.