Case definitions of the same phenomenon may be different for different purposes. Case definitions usually become more specific over time as more information about the condition becomes available. Idiopathic environmental intolerance is one of many labels for a heterogeneous group of conditions in which subjects describe multiple symptoms that are attributed to exposure to extremely low doses of common chemicals. Dr. Kreutzer presents issues in case definition for clinical purposes and for population-based studies, and makes recommendations for the clinician and for the public health investigator.