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In the latest updated ASCO/CAP guideline, a new category, ER low-positive was introduced, which are determined by the 1% to10 % of positive staining of the tumor nuclei for ER. However clinical meaning of ER-low positive was unclear compared with ER- positive or ER-negative in HER2 negative breast cancer. In order to clarify the clinicopathological features of ER-low positive breast cancer, we compared the clinicopathological factors and prognosis among three groups, ER negative, ER-low positive, and ER-positive groups in the patients with HER2 negative breast cancer. A total of 1,882 patients with HER2 negative breast cancer who underwent surgery between 2001 and 2018 were included in this study. We divided them into three groups, ER negative (According to ER status, 553(29.4%), 183(9.7%), 1146(60.9%) patients were divided into ER negative, low-positive and positive groups. Compared with ER-positive group, patients in the ER negative and low-positive groups were of higher age (p This study showed that the prognosis of the patients with ER negative and ER low-positive were similar, and poorer than that of those with ER positive tumors. Therefore, the new category of ER-low positive is clinically especially important in order to determine the appropriate adjuvant therapy. Citation Format: Tajiri Wakako. The clinical importance of the new category, ER low-positive in the ER expression HER2 negative early breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-05-11.