摘要The practice of outpatient breast cancer surgery has been controversial in the United States. This study aimed to update time trends and geographic variation in outpatient breast cancer surgery among eldedy Medicare fee-for-service women in the United States. Using the 1993-2002 linked Surveillance, Epidemiology and End Results (SEER)-Medicare claims data and the Area Resource Files, we identified 2 study samples, including the women whose breast cancers were the first-ever-diagnosed cancer at age 65 years or older from 9 regions continuously covered by the SEER registries since 1993. The first sample included the women receiving unilateral mastectomy for stage O-Ⅳ cancer; the second included the women receiving the breast-conserving surgery with lymph node dissection (BCS/LND) for stage O-Ⅱ cancer. The proportions of patients receiving outpatient surgery increased from 3.2% to 19.4% for mastectomy and from 48.9% to 77.8% for BCS/LND from 1993 to 2002. We observed substantial geographic variation in the average proportion of the patients receiving outpatient surgery in the studied areas across the 10-year period, ranging from 3.9% in Connecticut to 27.2% in Utah for mastectomy and from 54.7% in Hawaii to 78.1% in Seattle, Washington, for BCS/LND. As the popularity of outpatient breast cancer surgery continues to grow, more evidence-based analyses related to quality and outcomes of outpatient breast cancer surgery among various populations are needed in order to facilitate the public debates about state and federal mandated health benefit legislations.