BACKGROUND: Data are needed on how life expectancy affects treatment decisions among women ≥80 years with early stage breast cancer. METHODS: We used the linked Surveillance Epidemiology and End Results-Medicare claims dataset from 1992-2005 to identify women aged ≥80 newly diagnosed with lymph node negative, estrogen receptor positive tumors, ≤5 centimeters. To estimate life expectancy, we matched these women to women of similar age, region, and insurance, not diagnosed with breast cancer. We examined 5-year mortality of matched controls by illness burden (measured with the Charlson Comorbidity Index [CCI]) using Kaplan-Meier statistics. We examined treatments received by estimated life expectancy within CCI levels. We further examined factors associated with receipt of radiotherapy after breast conserving surgery (BCS). RESULTS: Of 9,932 women, 39.6% underwent mastectomy, 30.4% received BCS plus radiotherapy, and 30.0% received BCS alone. Estimated 5-year mortality was 72% for women with CCIs of 3+, yet 38.0% of these women underwent mastectomy and 22.9% received radiotherapy after BCS. Conversely, estimated 5-year mortality was 36% for women with CCIs of 0 and 26.6% received BCS alone. Age 80-84, urban residence, higher grade, recent diagnosis, mammography use, and low comorbidity, were factors associated with receiving radiotherapy after BCS. Among women with CCIs of 3+ treated with BCS, 36.9% underwent radiotherapy. CONCLUSIONS: Many women aged ≥80 with limited life expectancies receive radiotherapy after BCS for treatment of early stage breast cancers while many in excellent health do not. More consideration needs to be given to patient life expectancy when considering breast cancer treatments. KEY WORDS: Breast cancer, older women, treatment, life expectancy, radiation.