This study was designed to address whether simultaneous primary chemo-hormonal therapy provides additional activity compared with chemotherapy alone in breast cancer patients with operable or locally advanced disease. Between January 1997 and January 2002, 211 consecutive patients with T2-4, N0-1, M0 breast cancer were randomized to receive either epirubicin alone (EPI) or epirubicin plus tamoxifen (EPI-TAM). Ki67 expression was evaluated immunohistochemically in tumor specimens obtained before chemotherapy by incision biopsy and at definitive surgery. Tumor shrinkage of >50% was obtained in 76% of patients randomized in the EPI arm and 81.9% of patients randomized in the EPI-TAM arm (not significant). The corresponding rates of clinical and pathological complete response were 20.2 and 21.9% (not significant), and 4.8 and 6.7% (not significant), respectively. Pathologically complete response was more frequently observed in estrogen receptor (ER)-negative (ER-) tumors (P=0.04) and correlated with elevated baseline Ki67 expression (P<0.01). Both EPI and EPI-TAM treatments resulted in a significant reduction in Ki67 expression, either in overall patients (P=0.000) or in patients with ER+ breast cancer (P=0.000). The reduction in Ki67 immunostaining in the EPI-TAM arm was greater than in the EPI arm, leading to a lower Ki67 expression at post-operative residual histology (P=0.0041). The addition of tamoxifen to epirubicin chemotherapy did not improve the response rate but led to a significantly higher reduction in the Ki67 expression. Baseline elevated Ki67 expression and the ER- status were both associated with a greater chance of obtaining a pathological complete response at residual histology.