In the physicians's practice in past decades, an increasing number of patients suffering from polysomatic complaints with a subjective feeling of allergy against environmental noxious agents have been seen. Various names for this condition include "Eco-Syndrome" or "Multiple Chemical Sensitivity" (MCS), "Multiorgan Dysesthesia" or ''Idiopathic Environmental Intolerances". The uncertainty in the nomenclature reflects the deficiency in the knowledge of the etiopathophysiology and accordingly the diagnostic and therapeutic procedures. Most patients have completed an odyssey of visiting various specialists including psychiatrists and undergone many kinds of so-called alternative or parascientific procedures. We studied such patients since the early 1980s performing intensive interdisciplinary and allergological investigations. In about two third of the patients psychiatric or psychosomatic disturbances were obvious, but in one third of the patients somatic pathophysiological conditions were regarded as predominant cause of the present complaints. Many patients exhibited various pathophysiological patterns including somatic and psychosomatic alterations. Measurements of indoor air pollutants in the dust or in the air showed in some cases increased values of aromatic hydrocarbones, terpenes etc., without, however, explaining the main complaints. We conclude from our experience that patients presenting with hypersensitivity phenomena related to indoor air pollution are a heterogeneous group. There is no evidence, that "MCS" really exists, often it is diagnosed by the patients themselves. The term "eco-syndrome" describes a "working diagnosis" in order to apply careful interdisciplinary investigations for this heterogeneous group of patients. For the practical management of these patients mutual confidence is a prerequisite for success. The role of indoor air pollutants in triggering unspecific complaints beyond the exclusively toxicological field remains to be elucidated by future studies.