OBJECTIVES: Epidemiologic studies report that lack of adjuvant radiation (RT) after breast conserving surgery (BCS) is associated with higher short-term mortality. It is generally accepted that adjuvant RT decreases risk of breast cancer recurrence and thereby lowers long-term mortality; here, we explore reasons for its relationship to short-term mortality. MATERIALS AND METHODS: We studied 1,583 publically insured women who had BCS between 1998 and 2002 (mean 71.8 years, range 27-101), of whom 1,346 (85%) received RT. Multivariate analyses with Cox Proportional Hazards and Logistic Regression models included: age; race; comorbidity; insurance status; tumor size; number of nodes positive; hormone receptor status; receipt of radiation; adjuvant chemotherapy; preventive care - including mammography, Pap smear and primary care visits; and hospitalization. RESULTS: At a mean follow-up of 52.8 months, overall mortality was significantly lower in those who received RT (HR 0.45, p<0.0001) and higher with older age (HR 1.05, p<0.0001) and greater comorbidity (HR 1.16, p=0.0007). Local recurrence was less with receipt of optimal radiation (HR 0.47; p=0.03). Breast cancer event, as determined by a clinically logical algorithm to detect breast cancer recurrence and death, however, was not significantly associated with receipt of RT (OR 1.32, p=0.2). CONCLUSION: These results imply that the higher short-term mortality in women not receiving RT after BCS is related to factors other than breast cancer recurrence.