Background: The indication of adjuvant chemotherapy for patients with highly proliferative estrogen receptor-positive breast cancer is controversial. We analyzed the predictive value of Ki67 for the efficacy of adjuvant chemotherapy in patients with estrogen receptor-positive, node-positive breast cancer. Patients and methods: We identified 1241 patients with Luminal B early stage breast cancer with 1-3 axillary positive nodes who underwent surgery between 1995 and 2005 at the European Institute of Oncology and received adjuvant hormonotherapy and/or chemotherapy. Differences in the distribution of characteristics according to treatment were evaluated by the Chi-square test. To evaluate the effect of adding chemotherapy to hormonotherapy, the propensity score method was used to match patients' characteristics minimizing bias related to the non-random assignment of treatment. Results: The probability of receiving chemotherapy was significantly associated with age, tumor grade, degree of hormone responsiveness, tumor size and peripheral vascular invasion. The propensity score distribution was statistically different between the two treatment groups ( p