- Environmental factors such as ambient air pollution have been associated with congenital heart defects. The aim of this study was to investigate the association between gestational exposure to air pollution and the risk of congenital heart defects. We conducted a registry-based cohort study with a total of 135,527 live- and still-births in the Tel-Aviv region during 2000–2006. We used a Geographic Information System-based spatiotemporal approach with weekly inverse distance weighting modeling to evaluate associations between gestational exposure to ambient air pollution during weeks 3–8 of pregnancy and the risk for congenital heart defects. The following pollutants were studied: carbon monoxide, nitrogen-dioxide, ozone, sulfur-dioxide and particulate matter with aerodynamic diameter smaller than 10 μm and 2.5 μm (PM 10 , PM 2.5 respectively). Logistic models, adjusted for socio-demographic covariates were used to evaluate the associations. We found that maternal exposure to increased concentrations of PM 10 was associated with multiple congenital heart defects (adjusted OR 1.05, 95% CI: 1.01 to 1.10 for 10 μg/m 3 increment). An inverse association was observed between concentrations of PM 2.5 and isolated patent ductus arteriosus (adjusted OR 0.78, 95% CI: 0.68 to 0.91 for 5 µg/m 3 increment). Sensitivity analyses showed that results were consistent. Generally there were no evidence for an association between gaseous air pollutants and congenital heart defects.Our results for PM 10 and congenital heart defects confirm results from previous studies. The results for PM 2.5 need further investigations.