Background: Preterm births cause a large public-health burden, and air pollution is considered to be a potential risk factor. We evaluated the association between proximity to major roads (as an index for air pollution) and preterm births, classified by gestational age and specific clinical manifestations. Methods: Data on parental information and birth outcomes were extracted from the database maintained by the perinatal hospital in Shizuoka, Japan. We restricted the analysis to mothers who delivered liveborn single births from 1997 to 2008 (n = 14,226). Using the geocoded residential information, each birth was classified on its proximity to major roads. We estimated the multivariate-adjusted odds ratios and their 95% confidence intervals (CIs) for the association of proximity to major roads with preterm births, using logistic regression. Results: We found positive associations between proximity to major roads and preterm births at all gestational ages. Living within 200 m increased the risk of births before 37 weeks by 1.5 times (95% CI = 1.2-1.8), birth before 32 weeks by 1.6 times (1.1-2.4), and births before 28 weeks by 1.8 times (1.0-3.2). Proximity specifically increased th