Background: The EndoPredict 12-gene prognostic assay is validated to predict distant recurrence-free survival (DRFS) and response to chemotherapy in post-menopausal women with ER+, HER2- breast cancer. This study evaluated the performance of EndoPredict in pre-menopausal women. Methods: Stored tumor samples from women with ER+, HER2- primary breast cancer who were pre-menopausal at the time of diagnosis and were systemically treated with endocrine therapy alone were obtained from two sites (University of Nottingham and University of Cyprus). These samples were tested with EndoPredict to produce a 12-gene molecular score (EP). Pathologic tumor size and nodal status were algorithmically combined with the EP score to produce the clinicomolecular EPclin score. Cases with tumors > pT3, which were treated with chemotherapy, or for whom the EPclin score was missing or invalid were excluded. Associations of EP and EPclin with 10-year DRFS were evaluated in terms of hazard ratios (HRs) from Cox proportional hazards models stratified by cohort. 10-year DRFS was estimated for EPclin high-risk and low-risk women by Kaplan-Meier analysis. Results: Out of 411 eligible cases, 385 had a valid EPclin score and were included in the analysis. Mean age at breast cancer diagnosis was 46.5 years (standard deviation 4.7). Most women (N = 239, 62.6%) had grade II tumors and 16.1% (N = 62) had node-positive disease. Over the observation period (median 9.7 years, interquartile range 6.6-13.9 years), 35 women had a distant recurrence within 10 years. Both the molecular EP score and the molecular-clinicopathologic EPclin score were associated with increased risk of distant recurrence [HR 1.3, 95% confidence interval (CI) 1.2-1.5; p < 0.001 and HR 3.6, 95% CI 2.3-5.7; p < 0.001, respectively]. Of these patients, 249 (64.7%) were categorized as low risk by EPclin score while the remaining 136 (35.3%) were categorized as high risk. Compared to EPclin low-risk women, EPclin high-risk patients were more likely to experience distant recurrence (HR 4.6, 95% CI 1.4-15.2; p = 0.004). At 10 years post-diagnosis, EPclin low-risk women who received endocrine therapy alone had a DRFS of 97% (95% CI 93-99%). Conversely, EPclin high-risk women had a DRFS of only 76% (95% CI 67-82%). Conclusions: The EPclin score is highly associated with DRFS in pre-menopausal women who received adjuvant endocrine therapy alone. Based on these data, pre-menopausal women with EPclin low-risk breast cancer may safely forgo adjuvant chemotherapy in addition to endocrine therapy.