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Background In response to Choosing Wisely recommendations that sentinel lymph node biopsy (slnb) should not be routinely performed in elderly patients with node-negative (cN0), estrogen receptor-positive (er+) breast cancer, we sought to evaluate how nodal staging affects adjuvant treatment in this population. Methods From a prospective database, we identified patients 70 or more years of age with cN0 breast cancer treated with surgery for er+ her2-negative invasive disease during 2012-2016. We determined rates of, and factors associated with, nodal positivity (pN+), and compared the use of adjuvant radiation (rt) and systemic therapy by nodal status. Results Of 364 patients who met the inclusion criteria, 331 (91%) underwent slnb, with 75 (23%) being pN+. Axillary node dissection was performed in 11 patients (3%). On multivariate analysis, tumour size was the only factor associated with pN+ (p = 0.007). Nodal positivity rates were 0%, 13%, 23%, 33%, and 27% for lesions preoperatively sized at 0-0.5 cm, 0.5-1 cm, 1.1-2.0 cm, 2.1-5.0 cm, and more than 5.0 cm. Compared with patients assessed as node-negative, those who were pN+ were more likely to receive axillary rt (lumpectomy: 53% vs. 1%, p < 0.001; mastectomy: 43% vs. 2%, p < 0.001), and adjuvant systemic therapy (endocrine: 82% vs. 69%; chemotherapy plus endocrine: 7% vs. 2%, p = 0.002). Conclusions Of elderly patients with cN0 er+ breast cancer, 23% were pN+ on slnb. Size was the primary predictor of nodal status, and yet significant rates of nodal positivity were observed even in tumours preoperatively sized at 1 cm or less. The use of rt and systemic adjuvant therapies differed by nodal status, although the long-term oncologic implications require further investigation. Multidisciplinary input on a case-by-case basis should be considered before omission of slnb. [ABSTRACT FROM AUTHOR]