The effects of maternal smoking during pregnancy and childhood environmental tobacco smoke (ETS) exposure on asthma and wheezing were investigated in 5,762 school-aged children residing in 12 Southern California communities. Responses to a self- administered questionnaire completed by parents of 4th, 7th, and 10th grade students were used to ascertain children with wheezing or physician-diagnosed asthma. Lifetime household exposures to tobacco smoke were assessed using responses about past and current smoking histories of household members and any history of maternal smoking during pregnancy. Logistic regression models were fitted to cross-sectional data to estimate the effects of in utero exposure to maternal smoking and previous and current ETS exposure on the prevalence of wheezing and physician-diagnosed asthma. In utero exposure to maternal smoking without subsequent postnatal ETS exposure was associated with increased prevalence of physician-diagnosed asthma (OR, 1.8; 95% CI, 1.1 to 2.9), asthma with current symptoms (OR, 2.3; 95% CI, 1.3 to 4.0), asthma requiring medication use in the previous 12 mo (OR, 2.1; 95% CI, 1.2 to 3.6), lifetime history of wheezing (OR, 1.8; 95% CI, 1.2 to 2.6), current wheezing with colds (OR, 2.1; 95% CI, 1.3 to 3.4) and without colds (OR, 2.5; 95% CI, 1.4 to 4.4), persistent wheezing (OR, 3.1; 95% CI, 1.6 to 6.1), wheezing with exercise (OR, 2.4; 95% CI; 1.3 to 4.3), attacks of wheezing causing shortness of breath (OR, 2.4; 95% CI, 1.3 to 4.4) or awakening at night in the previous 12 mo (OR, 3.2; 95% CI, 1.7 to 5.8), and wheezing requiring medication (OR, 2.1; 95% CI, 1.2 to 3.7) or emergency room visits during the previous year (OR, 3.4; 95% CI, 1.4 to 7.8). In contrast, current and previous ETS exposure was not associated with asthma prevalence, but was consistently associated with subcategories of wheezing. Current ETS exposure was associated with lifetime wheezing (OR, 1.3; 95% CI, 1.1 to 1.5), current wheezing with colds (OR, 1.6; 95% CI, 1.3 to 2.0) and without colds (OR, 1.5; 95% CI, 1.1 to 1.9), wheezing with exercise (OR, 1.7; 95% CI, 1.3 to 2.2), attacks of wheezing causing shortness of breath (OR, 1.6; 95% CI, 1.2 to 2.1) or awakening at night (OR, 1.5; 95% CI, 1.1 to 2.0), and wheezing requiring medication (OR, 1.4; 95% CI, 1.1 to 1.8) or emergency room visits within the previous year (OR, 1.9; 95% CI, 1.2 to 3.0). The effects of current ETS exposure on subcategories of wheezing were most pronounced among children exposed to two or more smokers and remained significant after adjusting for maternal smoking during pregnancy. We conclude that maternal smoking during pregnancy increases the occurrence of physician-diagnosed asthma and wheezing during childhood. In contrast, current ETS exposure is associated with wheezing, but not physician-diagnosed asthma. Taken together, our findings support the hypothesis that ETS operates as a cofactor with other insults such as intercurrent infections as a trigger of wheezing attacks, rather than as a factor that induces asthma, whereas in utero exposure acts to increase physician-diagnosed asthma