BACKGROUND: Epidemiological evidence on the association between ambient air pollution and breast cancer risk is inconsistent. OBJECTIVE: We examined the association between long-term exposure to ambient air pollution and incidence of postmenopausal breast cancer in European women. METHODS: In 15 cohorts from nine European countries, individual estimates of air pollution levels at the residence were estimated by standardized land-usc regression models developed within the European Study of Cohorts for Air Pollution Effects (ESCAPE) and Transport related Air Pollution and Health impacts - Integrated Methodologies for Assessing Particulate Mattcr (TRANSPHORM) projects: particulate matter (PM) <= 2.5 mu m, <= 10 mu m, and 2.5-10 mu m in diameter (PM2.5, PM10, and PMcoarse, respectively); PM2.5 absorbance; nitrogen oxides (NO2 and NOx); traffic intensity; and elemental composition of PM. We estimated cohort-specific associations between breast cancer and air pollutants using Cox regression models, adjusting for major lifestyle risk factors, and pooled cohort-specific estimates using random-effects meta-analyses. RESULTS: Of 74,750 postmenopausal women included in the study, 3,612 developed breast cancer during 991,353 person-years of follow-up. We found positive and statistically insignificant associations between breast cancer and PM2.5 [hazard ratio (FIR) = 1.08 [95% confidence interval (Cl): 0.77, 1.51] per 5 mu g/m(3)}, PM10 [1.07 (95% CI: 0.89, 1.30) per 10 mu g/m(3)], PMcoarse [1.20 (95% Cl: 0.96, 1.49 per 5 mu g/m(3)], and NO2 [1.02 (95% CI: 0.98, 1.07 per 10 mu g/m(3)], and a statistically significant association with NOx [1.04 (95% CI: 1.00, 1.08) per 20 mu g/m(3), p = 0.04]. CONCLUSIONS: We-found suggestive evidence of an association between ambient air pollution and incidence of postmenopausal breast cancer in European women.