Since October 3, 2001, CDC and state and local public health authorities have been investigating cases of bioterrorism-related anthrax. This report updates findings as of October 31, and includes interim guidelines for the clinical evaluation of persons with possible anthrax. A total of 21 cases (16 confirmed and five suspected) of bioterrorism-related anthrax have been reported among persons who worked in the District of Columbia, Florida, New Jersey, and New York City (Figure 1). Until the source of these intentional exposures is eliminated, clinicians and laboratorians should be alert for clinical evidence of Bacillus anthracis infection. Epidemiologic investigation of these cases and surveillance to detect new cases of bioterrorism-associated anthrax continues.