Comorbid diseases can affect breast cancer prognosis. We conducted a population-based study of Danish women diagnosed with a first primary breast cancer from 1995 to 2005 (n=9300), using hospital discharge registry data to quantify comorbidities by Charlson score. We examined the influence of comorbidities on survival, and quantified their impact on relative mortality rates. The prevalence of patients with a Charlson score='0' fell from 86 to 81%, with an increase in those with Charlson score='1-2' from 13 to 16%, and score='3+' from 1 to 2%. One- and five-year survival for patients with Charlson score='0' and '1-2' was better for those diagnosed in 1998-2000 than in 1995-1997. Overall, patients diagnosed in 2001-2004 (mortality ratio (MR)=0.80, 95% CI=0.68-0.95) and 1998-2000 (MR=0.92, 95% CI=0.78-1.09) had lower 1-year age-adjusted mortality compared to those diagnosed in 1995-1997 (reference period). Patients with Charlson scores '1-2' and '3+' had higher age-adjusted 1-year mortality than those with a Charlson score='0' in each time period (2001-2004: MR('1-2')=1.76, 95% CI=1.35-2.30, and MR('3+')=3.78, 95% CI=2.51-5.68; and 1998-2000: MR('1-2')=1.60, 95% CI=1.36-1.88 and MR('3+')=2.34, 95% CI=1.65-3.33). Similar findings were observed for 5-year age-adjusted mortality. Additional analyses, adjusted for stage, indicated that confounding by stage could not explain these findings. Despite continued improvements in breast cancer survival, we found a trend of poorer survival among breast cancer patients with severe comorbidities even after adjusting for age and stage. Such poorer survival is an important public health concern and can be expected to worsen as the population ages.