The concept of an illness with subjective wide-ranging symptoms triggered by exposure to numerous environmental chemicals, called multiple chemical sensitivities (MCS), has existed for at least 40 years, but it remains controversial. No studies to date have been carried out to assess the prevalence of patients with the diagnosis of MCS in any community. The basis for the diagnosis of MCS has been examined by a number of investigators and found to be arbitrary and predicated solely on the fact or perception by the patient that a harmful environmental exposure had occurred and was responsible for subsequent symptoms. Theories of the etiology and pathogenesis of MCS are numerous and frequently change. They encompass physical, psychological and social concepts. Psychopharmacology in MCS treatment is frequently recommended, although without specific data on results. MCS has many features in common with other controversial syndromes such as the chronic fatigue syndrome.