ARTICLE ABSTRACTPurpose: We conducted a randomized phase III study to determine whether patients with early breast cancer would benefit from the addition of capecitabine (X) to a standard regimen of doxorubicin (A) plus cyclophosphamide (C) followed by docetaxel (T).Experimental Design: Treatment comprised eight cycles of AC→T (T dose: 100 mg/m2 on day 1) or AC→XT (X dose: 825 mg/m2 twice daily, days 1–14; T dose: 75 mg/m2 on day 1). The primary endpoint was 5-year disease-free survival (DFS).Results: Of 2,611 women, 1,304 were randomly assigned to receive AC→T and 1,307 to receive AC→XT. After a median follow-up of 5 years, the study failed to meet its primary endpoint [HR, 0.84; 95% confidence interval (CI), 0.67–1.05; P = 0.125]. A significant improvement in overall survival, a secondary endpoint, was seen with AC→XT versus AC→T (HR, 0.68; 95% CI, 0.51–0.92; P = 0.011). There were no unexpected adverse events. Of patients with estrogen receptor (ER)–positive/HER2-negative disease, 70% of whom were node-positive, 26% and 59% had tumors with a centrally assessed Ki-67 score of <10% or <20%, respectively, and only 17 (2%) and 53 (6%) DFS events, respectively, occurred in these groups at 7 years.Conclusions: The very low event rate in patients with ER-positive, low Ki-67 cancers, regardless of nodal status, strongly suggests that these patients should not be enrolled in adjuvant trials that assess 5-year DFS rates and that central Ki-67 analyses can identify these patients. Clin Cancer Res; 21(19); 4305–11. ©2015 AACR.