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Introduction Survival in breast cancer patients has steadily increased over the years, but with considerable disparities between individuals with different migration background and social position. We explored differences in diagnosis and all-cause mortality in breast cancer patients by stage of disease at the time of diagnosis and by country of birth, while considering the effect of comorbidity, regional and socio-demographic factors. Methods We used Swedish national registers to follow a cohort of 35,268 patients (4232 foreign-born) with breast cancer between 2004 and 2009 in Sweden. We estimated relative risk ratio (RRR) for diagnosis, hazard ratio (HR) for all-cause mortality and relative excess rate (RER) for breast cancer mortality using multinomial logistic regression models, multivariable Cox proportional hazard, and Poisson regression, respectively. Results We observed 4178 deaths due to any causes. Among them 418 women were born abroad. Foreign-born patients were on average 3 years younger at the time of breast cancer diagnosis and had higher risk of stage II tumors compared with Sweden-born women (RRR = 1.09, 95% CI 1.00–1.19). Risk of dying was 20% higher in foreign-born compared with Sweden-born breast cancer patients, if the tumor was diagnosed at stages III-IV after adjustment for age at diagnosis, education, county of residence and Charlson's comorbidity index (HR = 1.20, 95% CI 0.95–1.51 and RER = 1.21, 95% CI 0.95–1.55). Conclusions The worse prognosis in foreign-born patients with advanced tumors compared with Sweden-born patients is not explained by educational level or comorbidity. The reasons behind the observed disparities should be further studied.