Abstract: Objective : To explore the relationship between tumor volume and the expression of Bcl-2 protein before and after conventional or dose-dense chemotherapy in nude mice with MCF-7B and MCF-7 tumors. Methods : The experiment used transplanted MCF-7B tumors in which Bcl-2 was overexpressed and MCF-7 in which Bcl-2 was normally expressed. We observed the changes in tumor volume and the expression of Bcl-2 protein after conventional or dose-dense chemotherapy. Results : (1) After conventional or dose-dense chemotherapy, tumor volume was significantly decreased compared with that of the untreated control group. In mice with MCF-7B tumors, the tumor volume was decreased more in the dose-dense chemotherapy group than in the conventional chemotherapy group. No significant difference was found between the dose-dense chemotherapy group and the conventional chemotherapy group for MCF-7. (2) In mice with MCF-7B tumors, the expression of Bcl-2 protein in the dose-dense chemotherapy group and the conventional chemotherapy group was downregulated significantly, especially in the dose-dense chemotherapy group. (3) Multiple linear regression analyses showed that dose-dense chemotherapy and expression of Bcl-2 protein were correlated with changes in tumor volume. Conclusion : Transplanted MCF-7B tumors that overexpress Bcl-2 are more sensitive to dose-dense chemotherapy than transplanted MCF-7 tumors. Overexpression of Bcl-2 protein may play an important role in predicting the effect of dose-dense chemotherapy for breast cancer.