Nitrogen dioxide (NO(2)) is an important air pollutant due to its adverse effects on human health. Yet, current evidence on the association between NO(2) and the risk of breast cancer lacks consistency. In this study, we investigated the association between long-term exposure to NO(2) and breast cancer risk in the French E3N cohort study. Association of breast cancer risk with NO(2) exposure was assessed in a nested case-control study within the French E3N cohort including 5222 breast cancer cases identified over the 1990-2011 follow-up period and 5222 matched controls. Annual mean concentrations of NO(2) at participants' residential addresses for each year from recruitment 1990 through 2011, were estimated using a land use regression (LUR) model. Multivariable conditional logistic regression models were used to compute odds ratios (ORs) and their 95% confidence intervals (CIs). Additional analyses were performed using NO(2) concentrations estimated by CHIMERE, a chemistry transport model. Overall, the mean NO(2) exposure was associated with an increased risk of breast cancer. In all women, for each interquartile range (IQR) increase in NO(2) levels (LUR: 17.8 μg/m(3)), the OR of the model adjusted for confounders was 1.09 (95% CI: 1.01-1.18). The corresponding OR in the fully adjusted model (additionally adjusted for established breast cancer risk factors) was 1.07 (95% CI: 0.98-1.15). By menopausal status, results for postmenopausal women were comparable to those for all women, while no association was observed among premenopausal women. By hormone receptor status, the OR of estrogen receptor positive breast cancer = 1.07 (95% CI: 0.97-1.19) in the fully adjusted model. Additional analyses using the CHIMERE model showed slight differences in ORs estimates. The results of this study indicate an increased risk of breast cancer associated with long-term exposure to NO(2) air pollution. Observing comparable effects of NO(2) exposure estimated by two different models, reinforces these findings.Read less <