Background: In HR+/HER2- N0 early breast cancer (EBC), patients (pts) with Recurrence Score™ (RS) Methods: 5625 pts were registered and 4691 finally treated by ET (n=2356) or CT (n=2335) within ADAPTHR+/HER2-. ET-trial ITT population comprised 2290 pts: n=868 RS0-11, n=1422 RS12-25/ET-response(30% premenopausal, 26% N1). 5y-iDFS was 93.9% (95%-CI: [91.8% to 95.4%]) in RS0-11 and 92.6%(95%-CI: [90.8% to 94.0%]) in RS12-25/ET-responders. Since the one-sided upper 95% confidence limitof the 5y-iDFS difference was 3.3%, the pre-specified criterion to accept the primary NI-hypothesis wasmet (p=.05).5y-dDFS was 96.3% [94.6% to 97.5%] vs. 95.6% [94.2% to 96.7%]) in RS0-11 vs. RS12-25/ET-responders,respectively (95%-CI for 5y-dDFS difference: [-1.2% to 2.6%]). 5y-OS was also excellent and similar(98.0% [96.7% to 98.9%] vs. 97.3% [96.1 to 98.1%]) in RS0-11 vs. RS12-25/ET-responders, respectively(95% CI for the 5y-OS difference: [-0.7% to 2.2%]). Results: 5625 pts were registered and 4691 finally treated by ET (n=2356) or CT (n=2335) within ADAPTHR+/HER2-. ET-trial ITT population comprised 2290 pts: n=868 RS0-11, n=1422 RS12-25/ET-response(30% premenopausal, 26% N1). 5y-iDFS was 93.9% (95%-CI: [91.8% to 95.4%]) in RS0-11 and 92.6%(95%-CI: [90.8% to 94.0%]) in RS12-25/ET-responders. Since the one-sided upper 95% confidence limit of the 5y-iDFS difference was 3.3%, the pre-specified criterion to accept the primary NI-hypothesis was met (p=.05) 5y-dDFS was 96.3% [94.6% to 97.5%] vs. 95.6% [94.2% to 96.7%]) in RS0-11 vs. RS12-25/ET-responders, respectively (95%-CI for 5y-dDFS difference: [-1.2% to 2.6%]). 5y-OS was also excellent and similar (98.0% [96.7% to 98.9%] vs. 97.3% [96.1 to 98.1%]) in RS0-11 vs. RS12-25/ET-responders, respectively (95% CI for the 5y-OS difference: [-0.7% to 2.2%]). Conclusions: In pN0-1 luminal EBC pts receiving ET alone, pts with RS12-25/ET-response had 5y-iDFSwell above 90% and very close to RS0-11 pts. Both groups had excellent similar dDFS and OS. DynamicET response thus complements RS as a key selection criterion for omission of chemotherapy in pN0-1HR+/HER2- EBC. Citation Format: Nadia Harbeck, Oleg Gluz, Sherko Kuemmel, Matthias Christgen, Michael Braun, Bahriye Aktas, Kerstin Luedtke-Heckenkamp, Helmut Forstbauer, Eva-Maria Grischke, Claudia Schumacher, Maren Darsow, Katja Krauss, Benno Nuding, Marc Thill, Jochem Potenberg, Christoph Uleer, Mathias Warm, Hans H. Fischer, Wolfram Malter, Michael Hauptmann, Ronald Kates, Monika Graeser, Rachel Wuerstlein, Steve Shak, Rick Baehner, Hans Kreipe, Ulrike Nitz, West German Study Group. Endocrine therapy alone in patients with intermediate or high-risk luminal early breast cancer (0-3 lymph nodes), Recurrence Score