Multiple chemical sensitivities (MCS) defies diagnostic categorization because of its pansystemic manifestations, its lack of consistent symptomatology, and its absence of specific, measurable endpoints, either physical stigmata or laboratory findings. Controlled studies have been few and difficult to perform. Moreover, the phenomenon resists systematic investigation for two reasons. First, it has coalesced into a movement championed by zealous proponents and organized adherents. Second, it has gained support in the courts, the legislatures, and the regulatory agencies which have provided special protection and monetary awards to this phenomenon before science and medicine have defined it as a medical disorder. One hotly contested issue in the MCS debate is the relative role of organic versus psychological contribution to the symptoms of MCS patients. This is a critical issue most importantly because it determines approaches to treatment, secondarily because it affects regulatory action, disability determination, and compensability. The interest of the ISRTP in this phenomenon derives from the significant regulatory impact of this diagnosis on OSHA standards, EPA regulations, and FDA standards. The symposium was convened to explore the quality of the science underlying MCS with respect to diagnosis, and, particularly, to cause. The general objective of the symposium was to discuss the state of the science regarding MCS and its implications in regulatory toxicology and public health. The panel of speakers represented a variety of disciplines including toxicology, immunology, occupational medicine, psychiatry, psychology, epidemiology, and public health. The symposium was attended by a diverse audience representing the medical, scientific, business, legal, and regulatory communities. The majority of speakers critically questioned the characterization of MCS as a clinical entity, in light of historical comparisons to earlier false diagnoses, toxicological implausibility, and clinical inconsistencies.