Traffic emissions are a major source of air pollution in urban areas. The results of recent studies have suggested that air pollution may be related to adverse pregnancy outcomes such as low birth weight and preterm delivery. The authors investigated the association between traffic-related air pollution and preterm deliveries. The study group included all first-parity singleton live births that occurred during 1992-1997 to women who lived in a zone along the Zhong-Shan Freeway in Taiwan (N = 6,251). The zone was defined as areas 1,500-m wide on either side of the freeway; it was located in the San-Ming, Nan-Tzu, Chienchen, and Linya wards-all of which are residential areas in East Kaohsiung. The prevalence of deliveries of preterm birth infants was significantly higher among mothers who lived within 500 m of the freeway than among mothers who resided 500-1,500 m from the freeway. In their analysis, the authors controlled for several confounders (e.g., maternal age, season, marital status, maternal education, infant gender). The adjusted odds ratio was 1.30 (95% confidence interval = 1.03, 1.65) for delivery of preterm infants born to mothers who lived within 500 m of the freeway. Such data provide additional support for the hypothesis that air pollution can affect the outcome of pregnancy.