When human illness results from an unintentional or intentional release of a toxin (chemicals produced by metabolism in an organism [e.g., ricin]) or a toxicant (natural or synthetic chemicals not metabolically produced by an organism [e.g., nerve agents]) into the environment, uniform reporting is necessary to direct appropriate resources, assess the extent of morbidity and mortality, track poisoned persons, and monitor response to intervention. In this report, CDC presents case definitions to facilitate uniform reporting among local, state, and federal public health agencies of illness resulting from a chemical release. The report also explains the rationale for the structure of the case definitions, the audience for whom it is intended, the setting in which the case definitions might be used, and reasons each chemical presented in the report was selected. Clinical knowledge and diagnostic tools (e.g., biologic laboratory tests) for detecting chemical poisoning are likely to improve over time. CDC will create new case definitions and revise existing definitions to meet the needs related to emerging threats and to enhance case definition sensitivity and specificity, when possible, with developing clinical information.