Passive smoking, or environmental tobacco smoke (ETS), has been found to be causally associated with a large number of diseases in various organs although the evidence is sometimes conflicting. This review summarises the effects of passive smoking on respiratory symptoms, lung function and asthma in children and adults. In children, prenatal exposure to ETS is associated with impaired lung function and increased risk of developing asthma, while postnatal exposure mainly acts as a trigger factor for respiratory symptoms and asthma attacks. In adults, ETS exposure is associated with respiratory symptoms, asthma, a small but significant impairment of lung function and increased bronchial responsiveness. The consequence of workplace exposure seems to be more serious than domestic exposure. Legislative measures banning smoking at work have positive health effects in non-smokers and increase the quitting rate in smokers. Measures aimed at reducing childhood exposure to ETS should have high priority. Smoke cessation programmes for pregnant women attending antenatal clinics and for parents at the time of child hospitalisation for respiratory illness seem to have a fairly high success rate. Passive smoking is a widespread, important and avoidable risk factor for respiratory symptoms in both children and adults. Reducing passive smoking in the community will have a large positive effect on respiratory health.