Both in utero exposures to maternal smoking and asthma are associated with chronic deficits in lung function. We hypothesized that in utero exposure affects lung function in children without asthma and synergistically affects children with early onset asthma. To investigate effects of in utero exposure and age at asthma diagnosis on lung function, we examined longitudinal medical history, tobacco smoke exposure, and lung function data from 5,933 participants in the Children's Health Study. We found that children exposed in utero, but without asthma, showed decreased FEV1/FVC, FEF25-75, and FEF25-75/FVC ratio. Among children without in utero exposure, early asthma diagnosis was associated with larger decreases in FEV1, FEF25-75, and FEV1/FVC ratio compared with later diagnosed asthma. Children with in utero exposure alone and early onset asthma showed deficits in FEV1 (-13.6%; 95% confidence interval [CI], -18.9 to -8.2) and FEF25-75 (-29.7%; 95% CI, -37.8 to -20.5) among boys; and FEF25-75 (-26.6%; 95% CI, -36.4 to -15.1) and FEV1/FVC (-9.3%; 95% CI, -12.9 to -5.4) among girls. The absolute differences in FEF25-75 associated with in utero exposure increased with age in children with early onset asthma. We found little evidence for effects from environmental tobacco smoke exposure alone. In summary, deficits in lung function were largest among children with in utero exposure and early onset asthma.