Abstract: The purpose of this presentation is to discuss the possibility that sensitization is a psychobiological mechanism underlying not only multiple chemical sensitivity (MCS), but a much more general cluster of illness, referred to as “subjective health complaint. Sustained arousal, or sustained “stress” responses, may be an important factor for the development of these conditions. Patients with subjective complaints without objective changes are sometimes referred to as having “fashionable diagnoses” or “unexplained symptoms”. They may be given diagnoses like MCS, epidemic fatigue, chronic fatigue syndrome, burnout, stress, a variety of intoxications, environmental illness, radiation, multiple chemical hypersensitivity, food intolerance, functional dyspepsia, irritable bowel, myalgic encephalitis, postviral syndrome, yuppie flu, fibromyalgia, or vital exhaustion. One issue is whether this is one general condition or separate entities. Another issue is whether sensitization may be the psychobiological mechanism for most or all of these conditions. Finally, is it likely that sustained arousal may facilitate the development of sensitization in some or many neural circuits? In this review, the main emphasis will be on musculoskeletal pain. This is the most frequent and most expensive condition for sickness compensation and disability. The comorbidity of other complaints, however, will also be taken into account.