Background: Young age at diagnosis is an independent risk factor for recurrence and death from breast cancer (BC), with the greatest impact of young age demonstrated in hormone receptor positive (HR+) disease. Younger women are less likely to be adherent to endocrine therapy (ET), which may contribute to disparate outcomes. Methods: As part of a prospective cohort that enrolled women with BC diagnosed at age ≤40 between 2006-2016, we identified women with HR+, Stage I-III BC. Serial surveys with items assessing socio-demographic and treatment information including medication use are administered 1-2 times per year. Medical record review was used to ascertain stage and HR status and to confirm recurrent disease (locoregional, distant, or new primary breast cancer). Women who initiated ET but discontinued it Results: Among 607 women who initiated ET (median age at diagnosis: 36, range:17-40; 38%, 45%, 17% were Stage 1, 2, and 3, respectively).16% (99/607) were non-persistent, of whom 30% (30/99) discontinued ET ≤2 years post-diagnosis and over half had discontinued (54%, 54/99) by 3 years. In multivariable regression, those who were younger at diagnosis (age ≤30 vs. 36-40: OR: 3.39, 95% CI: 1.84-6.24; age 31-36 vs. 36-40: OR: 2.81 95% CI: 1.70-4.64) were more likely to discontinue ET while those with a higher stage at diagnosis were less likely to discontinue ET (Stage 2 vs. 1: OR: 0.46, 95% CI: 0.29-0.74; Stage 3 vs 1: OR: 0.32, 95% CI: 0.15-0.68). At a median follow-up time from diagnosis of 7.8 years (range 1-13 years), 15% of women (88/607) recurred or developed a new primary BC at a median time from diagnosis of 3.5 years (range 1-12 years). Of these, 66% (58/88) were distant recurrences, 32% (28/88) loco-regional, and 2% (2/88) new primary BCs. Rates of recurrence were higher among women who were persistent (81/508, 16%) vs. non-persistent (7/99, 7%, p=0.02). In multivariable regression, those who were non-persistent were less likely to recur (OR: 0.43, 95% CI: 0.19-0.98) while those with a higher stage were more likely to recur (Stage 3 vs. 1: OR: 2.36, 95% CI: 1.03-5.41). Sociodemographic, patient, and other treatment factors did not predict recurrence. Conclusions: Approximately 16% of young women with HR+ BC stop ET earlier than indicated, however non-persistence was not a risk factor for BC recurrence a median of 8 years following diagnosis in our cohort. Importantly, women with higher risk disease who are likely to benefit most from adherence to ET were less likely to discontinue treatment. Extended follow-up will further inform the impact of non-persistence on the incidence of late recurrences for which women with HR+ BC are at increased risk. Citation Format: Shoshana M Rosenberg, Yue Zheng, Philip Poorvu, Kathryn Ruddy, Rachel Gaither, Rulla Tamimi, Lidia Schapira, Jeffrey Peppercorn, Steven Come, Virginia Borges, Ann Partridge. Endocrine therapy non-persistence and recurrence in young women with early stage breast cancer [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 PD2-08.