The term "sick building syndrome" (SBS) has been used for several years to describe a number of mostly unspecific complaints of some occupants of air-conditioned buildings. Based on the literature, a definition of the SBS is given and some results of larger SBS studies carried out in Germany, Denmark and the United Kingdom are described. These studies show that SBS is a multifactorial event which may include physical, chemical, biological as well as psychological factors. In many cases, the occurrence of SBS in a building is due to insufficient maintenance of the HVAC system. SBS-induced investigations in a building should be carried out stepwise, the measurement of indoor air pollutants not being the first step. Strategies to avoid SBS must take into account factors specific to the building, the HVAC system and the equipment as well as psychological factors.