Aims: The aim was to assess all-cause and breast cancer mortality by income among women with incident breast cancer and the effect of cancer stage at diagnosis and comorbidity on the differences. Methods: The 43,439 women (age >=30) diagnosed with breast cancer in 1998–2008 in Finland were monitored using individual-level Cancer Registry data supplemented with data from Finnish health care registers and sociodemographic data. Results: Overall mortality was greater among breast cancer patients of the lowest income group than in the highest one; the hazard ratio (HR) for age and incidence year adjusted all-cause mortality was 1.9 (95% CI 1.8–2.1) and for breast cancer mortality 2.0 (95% CI 1.8–2.2). The women from the lowest income group were over-represented in the breast cancer stage ‘distant’ (10.1% vs. 3.5% among the highest income group). The presence of comorbidities was more common in patients with the lowest income (at least one comorbidity in 54%) than in the highest (24%). The HR adjusted with stage at presentation and comorbidities was 1.6 (95% CI 1.4–1.7) for all-cause mortality and 1.6 (95% CI 1.4–1.7) for breast cancer mortality. Conclusions: Tumour stage at diagnosis was more severe among lower income groups, and the presence of comorbidities was more common, although this only accounted for a part of the higher breast cancer mortality in these groups. More information is needed about recognition, diagnosis and treatment of breast cancer to examine whether the socioeconomic differences of breast cancer mortality are related to care.