Exposure to environmental tobacco smoke (ETS) is widespread in European countries, the most serious exposures occurring at home and in the workplace. Epidemiological studies have, essentially, addressed the association between ETS exposure and respiratory health in children, and increased risk of lung cancer among adult nonsmokers. Relatively few studies have been reported on ETS and adult non-neoplastic respiratory diseases. On the basis of the available data, no definite conclusion (excluding the acute irritating effect of ETS on respiratory mucous membranes) can be drawn. Although biologically plausible, it remains controversial whether ETS exposure is associated with chronic respiratory symptoms and occurrence of chronic obstructive pulmonary disease, including asthma. Most of the studies that have used the most sensitive indicators of pulmonary function have suggested a negative impact of ETS exposure. However, if really present, the physiological significance of such small changes is unclear, and the relationship to long-term changes in lung function is not established. Moreover, the possibility of bias and confounding factors must be taken into account. Thus, there is a need for further epidemiological studies on ETS exposure and adult non-neoplastic respiratory disorders.