The prevalence of coexistent diseases in addition to breast cancer becomes increasingly important in an ageing population. However, the clinical implications are unclear. The age-specific prevalence of serious comorbidity among all new breast cancer patients diagnosed from 1995 to 2001 (n=8966) in the South of the Netherlands was analysed in relation to age, stage and treatment. Independent prognostic effects of age and comorbidity were evaluated (follow-up was continued until 1 January 2004). The prevalence of comorbidity increased from 9% for those aged <50 years to 56% for patients aged 80+ years. The most frequent conditions were cardiovascular disease (7%), diabetes mellitus (7%), and previous cancer (6%). In the presence of comorbidity, fewer patients received radiotherapy (51% vs. 66%, P<0.0001) and fewer patients who underwent breast-conserving surgery also had axillary dissection (P<0.0001). Relative 5-year survival rates for patients without comorbidity (87%) were significantly higher (P<0.01) than those for patients with previous cancer (77%), diabetes mellitus (78%), and for patients with 2+ coexistent diseases (59%). Relative survival of patients without comorbidity increased with age to 93% for patients older than 70 years. Comorbidity negatively affected prognosis, independent of age, stage of disease, and treatment (Hazard Ratio (HR)=1.3, P=0.0001 for one coexistent disease and HR=1.4, P=0.0001 for 2+ coexistent diseases). The most important effects were found for previous cancer (HR=1.4, P=0.003), cerebrovascular disease (HR=1.6, P<0.004) or dementia (HR=2.3, P<0.0001). Elderly breast cancer patients can be divided in those without other diseases, who have a relatively good prognosis, and those who have at least one other serious coexistent disease and significantly poorer prognosis.