Description: Objectives Atmospheric fine particulate matter (PM 2.5 ) has multiple adverse effects on human health. Global atmospheric levels of PM 2.5 increased by 0.55 μg/m 3 /year (2.1%/year) from 1998 through 2012. There is evidence of a causal relationship between atmospheric PM 2.5 and breast cancer (BC) incidence, but few studies have investigated BC mortality and atmospheric PM 2.5 . We investigated BC mortality in relation to atmospheric PM 2.5 levels among patients living in Varese Province, northern Italy. Methods We selected female BC cases, archived in the local population-based cancer registry, diagnosed at age 50–69 years, between 2003 and 2009. The geographic coordinates of each woman's place of residence were identified, and individual PM 2.5 exposures were assessed from satellite data. Grade, stage, age at diagnosis, period of diagnosis and participation in BC screening were potential confounders. Kaplan-Meir and Nelson-Aalen methods were used to test for mortality differences in relation to PM 2.5 quartiles. Multivariable Cox proportional hazards modelling estimated HRs and 95% CIs of BC death in relation to PM 2.5 exposure. Results Of 2021 BC cases, 325 died during follow-up to 31 December 2013, 246 for BC. Risk of BC death was significantly higher for all three upper quartiles of PM 2.5 exposure compared to the lowest, with HRs of death: 1.82 (95% CI 1.15 to 2.89), 1.73 (95% CI 1.12 to 2.67) and 1.72 (95% CI 1.08 to 2.75). Conclusions Our study indicates that the risk of BC mortality increases with PM 2.5 exposure. Although additional research is required to confirm these findings, they are further evidence that PM 2.5 exposure is harmful and indicate an urgent need to improve global air quality.