In the present study, we investigated whether age and serious comorbid conditions influence treatment decisions, complications and survival in breast cancer patients. The Eindhoven Cancer Registry records patient, tumour and therapy characteristics of all patients diagnosed with cancer in the southern part of the Netherlands. Additional information on severity of comorbidity and serious complications was collected for a random sample of 527 breast cancer patients (aged 40 years and older). More than 70% of the patients >or=80 exhibited high severity of comorbidity compared to 6% of those aged 40-49 years. Treatment was not influenced by severity of comorbidity. Less than 30% of the breast cancer patients had complications after diagnosis. The number of complications was not related to age or severity of comorbidity. The hazard ratio (HR) of dying for patients with low/moderate severity of comorbidity was 2.4 for those aged 40-69 years and 1.6 for those aged >or=70 years, after adjustment for age, nodal status and treatment. For patients with high severity of comorbidity, the risk of dying was almost three times higher. Older breast cancer patients with serious comorbidity were not treated differently and did not have more complications compared to those without comorbidity, but they exhibited a worse prognosis.