Postmenopausal women aged 55 years and older have 66% of incident breast tumors and experience 77% of breast cancer mortality, but other age-related health problems may affect tumor prognosis and treatment decisions. To document the comorbidity burden of postmenopausal breast cancer patients and evaluate its relationship with age on disease stage, treatment, and early mortality. Data were collected on breast cancer patients' comorbidities by retrospective hospital medical records review and merged with information on patients' tumor characteristics collected from 6 regional National Cancer Institute Surveillance, Epidemiology, and End Results cancer registries. Patients were followed up until death or for 30 months from breast cancer diagnosis. Population-based random sample of 1800 postmenopausal breast cancer patients diagnosed in 1992 stratified by 3 age groups: 55 to 64 years, 65 to 74 years, and 75 years and older. Extent of disease, therapy received, comorbidity, cause of death, and survival. Seventy-three percent (1312 of 1800) of the sample was diagnosed with stage I and II breast cancer, 10% (n = 188) with stage III and IV breast cancer, and 17% (n = 300) did not have a stage assignment. Of the 1017 patients with stage I and stage II node-negative breast cancer, 95% received therapy in agreement with the National Institutes of Health consensus statement recommendation for early-stage breast cancer. Patients in older age groups were less likely to receive therapy consistent with the consensus statement (P<.001), and women aged 70 years and older were significantly less likely to receive axillary lymph node dissection as determined by logistic regression analysis (P<.01). Diabetes, renal failure, stroke, liver disease, a previous malignant tumor, and smoking were significant in predicting early mortality in a statistical model that included age and disease stage. Breast cancer was the underlying cause of death for 135 decedents (51.3%). Heart disease (n = 45, 17.1%) and previous cancers (n = 22, 8.4%) were the next major underlying causes. In the 30-month follow-up period, 263 patients (15%) died. Patient care decisions occur in the context of breast cancer and other age-related conditions. Comorbidity in older patients may limit the ability to obtain prognostic information (ie, axillary lymph node dissection), tends to minimize treatment options (eg, breast-conserving therapy), and increases the risk of death from causes other than breast cancer.