We investigated the association of major comorbidities with breast cancer outcomes using the Shanghai Breast Cancer Survival Study, a population-based, prospective cohort study of Chinese women diagnosed with breast cancer. Analyses included 4,664 women diagnosed with stage I-III incident breast cancer aged 20-75 years (median age = 51) during 2002-2006. Women were interviewed at 3-11 months post-diagnosis (median = 6.4) and followed up by in-person interviews and linkage with the vital statistics registry. Multivariable hazard ratios (HRs) and (95 % confidence intervals (CIs)) for the associations of comorbidities with breast cancer outcomes were estimated using Cox regression models. After a median follow-up of 5.3 years (range: 0.64-8.9), 647 women died (516 from breast cancer) and 632 recurrence/metastases were documented. The main comorbidities reported included: hypertension (22.4 %), chronic gastritis (14.3 %), diabetes mellitus (6.2 %), chronic bronchitis/asthma (5.8 %), coronary heart disease (5.0 %), and stroke (2.2 %). Diabetes was associated with increased risk of total mortality (adjusted HR: 1.40 (1.06-1.85)) and non-breast cancer mortality (adjusted HR: 2.64 (1.63-4.27)), but not breast cancer-specific mortality (adjusted HR: 0.98 (0.68-1.41)), adjusting for socio-demographics, clinical characteristics, selected lifestyle factors, and other comorbidities. Women with a history of stroke had a non-significant increased risk of total mortality (adjusted HR: 1.42 (0.91-2.22)) and a significant increased risk of non-breast cancer mortality (adjusted HR: 2.52 (1.33-4.78)), but not breast cancer-specific mortality (adjusted HR: 0.78 (0.38-1.62)). Overall, none of the comorbidities investigated were significantly associated with recurrence. In this large prospective cohort of breast cancer survivors, diabetes was significantly associated with increased risk of total and non-breast cancer mortality, and history of stroke was associated with increased risk of non-breast cancer mortality.