Background: Luminal HER2 negative breast cancer is a highly heterogeneous disease, which complicates the choice of optimal systemic treatment in early stage disease. It is known that in postmenopausal patients (pts) Ki67 level after short course of preoperative endocrine therapy (PET) correlates with survival in contrast to baseline level. We performed the trial of PET in premenopausal early stage breast cancer pts.Patients and methods: This is a non-randomized, open-label, single-arm, phase II study of short course of preoperative tamoxifen in premenopausal pts. Primary objective was a decline in Ki67 level below 10%, secondary objectives were biomarker changes and disease free survival. Between 2011 and 2017 74 pts with T1-2N0-1M0 ER+ HER2 negative breast cancer were included in the study, median age was 45 (range, 32-55), median baseline Ki67 30% (range, 5-96), median stromal TILs 5% (range, 0-40). All pts were treated with tamoxifen for 2-3 weeks before surgery. Median follow-up was 56 months (range, 29.6-124.3).Results: There was a statistically significant decline in Ki67 during the short course of endocrine therapy (p30% had post-PET Ki6730% - 92,6% (p=0.54). Also there was no difference in survival according to Ki67 level after the course of endocrine therapy: with post-PET Ki6730 – 94,4% (p=0.61). We explain this with the changes in adjuvant treatment in pts with high (>30%) level of Ki67 after endocrine therapy – all pts were receiving adjuvant chemotherapy and, what is more important, they also had ovarian suppression and tamoxifen has been changed to aromatase inhibitors (AI). Conclusion: to our knowledge this is the first study of the short course of PET in premenopausal pts. We demonstrated that post-PET Ki67 level can be used for individualization of adjuvant therapy (intensification with chemotherapy, ovarian suppression, AI in poor responders and avoiding of chemotherapy in good responders). Further prospective randomized trials are warranted. Citation Format: Mona Frolova, Marina Stenina, Alexander Petrovsky, Olga Krohina, Sergei Tjulandin. Short course of preoperative tamoxifen in premenopausal breast cancer patients: Biomarker changes and survival [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS5-38.