Passive smoking has a harmful effect on the respiratory health of children. Environmental tobacco smoke is composed of more than 3800 different chemical compounds, and concentrations of respirable suspended particulate matter can be two to three times higher in homes with smokers than in homes with no smokers. This statement reviews the evidence that children exposed to environmental tobacco smoke have higher rates of lower respiratory illness during their first year of life, higher rates of middle ear effusion, and higher rates of sudden infant death syndrome. In addition, children with asthma whose parents smoke have more severe symptoms and more frequent exacerbations.