T he potentially fatal effects of maternal smoking on infants have been recognised since the 1800s [1]. In the latter half of the 1900s, the link between maternal smoking and the increased rate of lower respiratory infection (LRI) in early life was described [2]. Given the high rates of smoking in many societies [3], the burden of the increase in LRI is substantial. In a meta-analysis of relationships between parental smoking and respiratory disease, LI et al. [4] calculated that the odds ratio (OR) for hospitalisation of LRI in infancy or early childhood for those exposed to parental environmental tobacco smoke (ETS) compared with those not exposed was 1.93 (95% confidence interval (CI) 1.66-2.25). The risk was age related, as the ORs of prevalence of serious LRI were 1.71 (95% CI 1.33-2.20) and 1.25 (95% CI 0.88-1.78) for children aged 0-2 and 3-6 yrs, respectively. LI et al. [4] concluded that the results of community and hospital studies were broadly consistent, and showed that the child of a parent who smokes is at approximately twice the risk of having a serious respiratory tract infection requiring hospitalisation in early life. Indeed, early respiratory infection is not only a major cause of morbidity in developed countries, but, in developing countries, respiratory infection is also the leading cause of death in children [5].There have been a large number of studies showing the relationships between maternal smoking and impaired lung function and increases in respiratory symptoms in infants and young children. An effect of maternal smoking on neonatal lung function has been demonstrated in a study performed very soon after birth [6]. In addition, maternal smoking has been associated with increased wheeze [7] in early life and in schoolchildren [8]. However, few studies have been large enough to separate the effects of placental-borne exposure during pregnancy and breast milk, and air-borne exposure after delivery. Studies that have been able to do this have concluded that the most important deleterious effects have occurred before birth. In a cohort study of 12,743 children from the UK, admission rates for lower respiratory diseases in children born to mothers who stopped smoking during pregnancy were as high as in children whose mothers smoked both during and after pregnancy [9]. Admission rates for lower respiratory diseases in children whose mothers smoked only post-natally were no higher than in those with nonsmoking mothers. Post-natal exposure exerted a significant influence on the incidence of bronchitis, but less than the effect of in utero exposure. For impairment in lung function in schoolchildren, similar conclusions regarding in utero versus post-natal exposure were reached by a more recent Californian study of 3,357 children [10]. In 5,762 subjects from the same Californian study, maternal smoking during, but not after, pregnancy was associated with an increased incidence of asthma [11].Thus, the previous studies have established that maternal smoking during pregnancy exerts a highly delete...