PURPOSE The indications of adjuvant chemotherapy for patients with estrogen receptor (ER) -positive breast cancer are controversial. We analyzed the predictive value of Ki67, HER2, and progesterone receptor (PR) expression for the efficacy of docetaxel in patients with ER-positive, node-positive breast cancer. PATIENTS AND METHODS Expression of Ki67, HER2, and PR was measured by immunohistochemistry in tumor samples from 798 patients with ER-positive breast cancer who participated in PACS01, a randomized trial that evaluated the efficacy of docetaxel. Risk reduction was evaluated using a Cox model adjusted for age, tumor size, nodal involvement, treatment arm, and biomarkers. The predictive value of biomarkers was assessed by an interaction test. Disease-free survival (DFS) was the primary end point. Results Ki67, HER2, and PR were expressed in 21%, 9%, and 62% of samples, respectively. Hazard ratios for relapse associated with docetaxel were 0.51 (95% CI, 0.26 to 1.01) in ER-positive/Ki67-positive tumors and 1.03 (95% CI, 0.69 to 1.55) in ER-positive/Ki67-negative tumors (ratio for interaction: 0.53; 95% CI, 0.24 to 1.16; P = .11). Five-year DFS rates were 81% (95% CI, 76% to 86%) and 84% (95% CI, 75% to 93%) in patients with ER-positive/Ki67-negative and ER-positive/Ki67-positive tumors treated with docetaxel and 81% (95% CI, 76% to 86%) and 62% (95% CI, 52% to 72%) in patients with ER-positive/Ki67-negative and ER-positive/Ki67-positive tumors treated with fluorouracil, epirubicin, and cisplatin. No trend for interaction was observed between docetaxel and HER2 (ratio for interaction: 0.83; 95% CI, 0.35 to 1.94; P = .66), nor between docetaxel and PR (ratio for interaction: 0.89; 95% CI, 0.47 to 1.66; P = .71). CONCLUSION Ki67 expression identifies a subset of patients with ER-positive breast cancer who could be sensitive to docetaxel treatment in the adjuvant setting.