Compelling evidence suggests a causal relation between parental smoking during and after pregnancy and adverse effects on respiratory health in the offspring. The authors' aim in this study was to disentangle the effects of prenatal and postnatal smoking on early childhood respiratory health. Most parents who smoke during pregnancy continue to smoke postpartum, and it is difficult to identify sufficiently large subgroups of children who were exclusively exposed in utero or after birth. This study was based on the first 22,390 children born between 2000 and 2004 in the Norwegian Mother and Child Cohort, a pregnancy cohort designed to eventually include 100,000 pregnancies. Data were collected through detailed questionnaires administered at various stages of pregnancy, starting in early pregnancy. Because of the large study population, the authors were able to disentangle the pre- and postnatal effects of parental smoking on wheeze and lower respiratory tract infection in the children's first 18 months of life. They found maternal smoking in pregnancy to be an independent risk factor for wheeze and respiratory infection. Postnatal paternal smoking was also associated with these outcomes, independently of maternal smoking in pregnancy.