Background: Nowadays, the right way for therapy decision-making in HR+ HER2- early breast cancer (BC) includes the complex of validated multigene assays (VMA) result with clinical and IHC evaluation. Given the inability to use VMA in routine clinical practice, the short-term pre-operative endocrine therapy (P-ET) allows to estimate more precisely plan the further adjuvant therapy (AT) in the patients (pts) with early HR+ HER2- BC. Our work is aimed to share of pts with P-ET benefit, or tumor phenotype reversion for proper treatment planning in pts with early HR+ HER2- BC. Methods: HR+ HER2- BC pts stages I-II any level of Ki67 and any grade (G) have received P-ET (letrozole (L)/anastrozole (A)/tamoxifen (TAM)) no less than 2 weeks until surgery in A.S. Loginov MCSC, Russia. Enrollment period was Jun’20 - Jan’23. The study was aimed to access P-ET efficiency in pts with baseline Ki67>10% downgrading to ≤10% (1), tumor ET sensitivity in the whole group (2) and/or phenotype transformation during P-ET followed by changing AT strategy (3). Results: At the data cut-off (Jan’23) 998 pts (5 pts were men) with HR+ HER2- BC were enrolled with median age 63,5 (range:26-85). The median time of P-ET was ≈19,5 days (range: 14-42). Pts flow according to tumor Ki67 level dynamic is show. 54% (434) pts with initially Ki67 level >10% decreased Ki67≤10% after P-ET in whom 36% (314) pts avoided adjuvant chemotherapy. In Ki67≤10% baseline group (n=187) tumor Ki67 level after P-ET increased >30% in every fifth case (n=35, 19%), that may reflect the initial ET resistance. Ki67 level dynamic change under P-ET was observed regardless of the TAM/IA use. 4% (38) pts has resulted the phenotype change to HER2+ BC (n=32) and TNBC (n=6) with subsequent AT decisions (anti-HER2-based or dose dense regimens, respectively). In this group there weren’t any Ki67 decrease after P-ET that additionally confirms the unmet need for treatment plan correction. Conclusions: In HR+ HER2- BC pts the results of P-ET with TAM or IA before surgery helps to estimate pts prognosis and further AT strategy: to identify the pts population of whom chemotherapy can be avoided while maintaining treatment efficacy with a better quality of life (1), to assess the initial ET tumor sensitivity (Ki67 baseline level independently) (2), to radically change the planned AT to achieve optimal treatment results (3). Mature data of long-term pts outcome are awaited.