From October 4 to November 2, 2001, the Centers for Disease Control and Prevention and State and local public health authorities reported 10 confirmed cases of inhalational anthrax and 12 confirmed or suspected cases of cutaneous anthrax. The outbreak resulted from intentional delivery of Bacillus anthracis spores through mailed letters or packages. The article includes medical history data, disease symptoms, and treatment for each of the 10 individuals. Their clinical presentation was variable and often resembled a viral respiratory illness, but the interpretation of the initial symptoms in the context of a possible exposure to Bacillus anthracis led to an early diagnosis. Survival may be markedly improved by combination antimicrobial therapy begun during the initial phase of the illness and by aggressive supportive care (e.g., drainage of pleural effusions). The article describes newer methods of detection such as polymerase chain reaction, immunohistochemistry, and sensitive serologic tests as important adjunctive diagnostic modalities that aid in the diagnosis of Bacillus anthracis infections. The article concludes that studies are needed to further define optimal antimicrobial regimens, explore the role of adjunctive therapies, and better understand the pathogenesis of inhalational anthrax associated with intentional release. Tables, figures, references