Recent studies have suggested that prenatal exposure to environmental tobacco smoke may lead to lower lung function in infants. The authors examined the relation of maternal smoking during pregnancy to persistent deficits in the lung function of older children. Subjects were 8,863 nonsmoking white children aged 8-12 years from 22 North American communities. Information on maternal smoking was provided by the child's mother. Pulmonary function testing of the children was conducted at school in 1988-1991. Children whose mothers smoked during pregnancy, whether or not they still smoked in the year prior to the study, had significantly lower lung function than did children whose mothers did not smoke in either period. On average, forced expiratory flow between 65 and 75 percent of forced vital capacity (FEF65-75%), forced expiratory flow between 25 and 75 percent of forced vital capacity (FEF25-75%), and forced expiratory volume in 3/4 of a second (FEV0.75) were 5.7%, 4.9%, and 1.7% lower, respectively, for children whose mothers smoked during pregnancy. After adjusting for maternal smoking during pregnancy, the authors found that current maternal smoking was not associated with significant differences on any lung function measure. These results show a persistent deficient in lung function associated with maternal smoking during pregnancy that is not explained by current maternal smoking alone. The strongest effects were observed with pulmonary function measures of flow in the small airways. The authors conclude that the effects of exposure to tobacco smoking by the mother during pregnancy and/or environmental tobacco smoke exposure in the first few years of life persist into childhood and may affect the pulmonary function attained throughout the child's life.