Hazardous materials incidents include gas and vapor releases, spills, explosions, and fires. Incidents involving human exposure are challenging for most health care providers because of the vast number of potential chemicals involved, frequently incomplete incident information, and limited experience in exposure assessment. To facilitate improved evaluation and treatment of patients with chemical exposures, the Washington Poison Center established the Hazardous Materials Exposure Information Service in 1994. During the first 33 months of operation, this service has provided information on 70 incidents, involving a total of 1120 exposed individuals, including 501 patients treated in medical facilities. This paper reviews these incidents, the process used to collect information from the incident scene, and selected techniques for evaluating the extent of individual chemical exposure.