The incidence of breast cancer, as well as other chronic disease, increases with age, older breast cancer patients being more likely than younger to suffer from other diseases at time of diagnosis. Our objective was to assess the effect of comorbidity on mortality after early breast cancer. 62,591 women diagnosed with early breast cancer 1990-2008 were identified using the Danish Breast Cancer Cooperative Group Registry. Data were linked to the Danish National Patient Register and the Danish Register of Causes of Death. Main outcome measures were mortality from all causes, breast cancer, and non-breast cancer causes in relation to Charlson comorbidity index (CCI). Compared with patients without comorbidity (CCI 0), the presence of comorbidity increased the risk of dying from breast cancer as well as other causes with adjusted hazard ratios (HRs) for all-cause mortality of 1.45 (CI 95% 1.40-1.51) for CCI 1, 1.52 (95% CI 1.45-1.60) for CCI 2, and 2.21 (95% CI 2.08-2.35) for CCI 3+. Equivalent HRs for breast cancer-specific mortality were 1.30 (95% CI, 1.24-1.36) for CCI 1, 1.31 (95% CI 1.23-1.39) for CCI 2, and 1.79 (95% CI, 1.66-1.93) for CCI 3+ (all P values < 0.0001). For patients with CCI 0, 5-year overall survival increased over time from 72.5% (95% CI, 71.7-73.3%) in 1990-1994 to 81.6% (95% CI, 80.9-82.2) in 2000-2004, whereas the 5-year overall survival remained stable around 43% among the patients with CCI 3+. This population-based cohort study shows that compared with patients without comorbidity, the risk of dying from breast cancer as well as other causes increased significantly with increasing CCI score. While survival improved over time for patients without comorbidity, no improvement was seen among patients with severe comorbidity (CCI 3+).