Purpose: To validate the clinical performance of the OncoMasTR Risk Score in the biomarker cohort of ABCSG Trial 8. Experimental Design: We evaluated the OncoMasTR test in 1200 formalin-fixed, paraffin-embedded surgical specimens from postmenopausal women with estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative primary breast cancer with 0-3 involved lymph nodes in the prospective, randomized ABCSG Trial 8. Time to distant recurrence (DR) was analyzed by Cox models. Results: The OncoMasTR Risk Score categorized 850 of 1087 (78.2%) evaluable patients as "low risk". At 10 years, the DR rate for patients in the low-risk group was 5.8% versus 21.1% for patients in the high-risk group (P<0.0001, absolute risk reduction 15.3%). The OncoMasTR Risk Score was highly prognostic for prediction of DR in years 0-10 in all patients (hazard ratio (HR) 1.91, 95% confidence interval (CI) 1.62 to 2.26, P<0.0001; C-index 0.73), in node-negative patients (HR 1.79, 95% CI 1.43 to 2.24, P<0.0001; C-index 0.72), and in patients with 1-3 involved lymph nodes (HR 1.93, 95% CI 1.44 to 2.58, P<0.0001; C-index 0.71). The OncoMasTR Risk Score provided significant additional prognostic information beyond clinical parameters, Ki67, Nottingham Prognostic Index, and Clinical Treatment Score. Conclusions: OncoMasTR Risk Score is highly prognostic for DR in postmenopausal women with ER-positive, HER2-negative primary breast cancer with 0-3 involved lymph nodes. In combination with prior validation studies, this fully independent validation in ABCSG Trial 8 provides level 1B evidence for the prognostic capability of the OncoMasTR Risk Score.