This review synthesizes current knowledge of the effects of prenatal and postnatal exposure to environmental tobacco smoke on the respiratory health of children. A Medline database search was conducted for 1966 through October 2000. Limited evidence was found that exposure in pregnancy influences fetal growth, increases the risk of preterm delivery, and predicts the development of asthma and reduced lung function later in life. Both occupational and home environments contribute to the exposure of pregnant women and thus indirectly to adverse effects on children. There is strong and consistent evidence that exposure in childhood causes chronic respiratory symptoms (eg, cough, phlegm, and wheezing) and induces asthma. Limited evidence supports the role of childhood exposure in the poor overall control of established disease. Postnatal exposure is likely to have a small adverse impact on lung function growth. Prenatal and postnatal exposures have an important impact on children's respiratory health. These effects are preventable if pregnant women and children are protected from exposure to environmental tobacco smoke.