Background: Early life is suggested to be a critical time in determining subsequent asthma development, but the extent to which the effect of early-life exposure to ambient air pollution on childhood asthma is unclear. Objectives: We investigated doctor-diagnosed asthma in preschool children due to exposure to ambient air pollution in utero and during the first year of life. Methods: In total 2490 children aged 3-6 years participated in a questionnaire study regarding doctor-diagnosed asthma between September 2011 and January 2012 in China. Children's exposure to critical air pollutants, sulfur dioxide (SO2) as proxy of industrial air pollution, nitrogen dioxide (NO2) as proxy of traffic pollution, and particulate matter <= 10 mu m in diameter (PM10) as a mixture, was estimated from the concentrations measured at the ambient air quality monitoring stations by using an inverse distance weighted (IDW) method. Logistic regression analysis was employed to determine the relationship between early-life exposure and childhood asthma in terms of odds ratio (OR) and 95% confidence interval (CI). Results: Association between early-life exposure to air pollutants and childhood asthma was observed. SO2 and NO2 had significant associations with adjusted OR (95% CI) of 1.45 (1.02-2.07) and 1.74 (1.15-2.62) in utero and 1.62 (1.01-2.60) and 1.90 (1.20-3.00) during the first year for per 50 mu g/m(3) and 15 mu g/m(3) increase respectively. Exposure to the combined high level of SO2 and NO2 in China significantly elevated the asthmatic risk with adjusted OR (95% CI) of 1.76 (1.18-2.64) in utero and 1.85 (1.22-2.79) during the first year compared to the low level exposure. The associations were higher for males and the younger children aged 3-4 than females and the older children aged 5-6. Conclusions: Early-life exposure to ambient air pollution is associated with childhood asthma during which the level and source of air pollution play important roles. The high level and nature of combined industrial and traffic air pollution in China may contribute to the recent rapid increase of childhood asthma. (C) 2015 Elsevier Inc. All rights reserved.