Cyclin D1 expression is closely related with ER status in breast cancer. We executed this study to evaluate whether therapeutic response to tamoxifen varies with levels of cyclin D1 expression in 66 ER positive breast cancer patients having solitary bone metastasis. Treatment response to tamoxifen and correlation between cyclin D1 expression and biologic data of the patients were analyzed. Cyclin D1 expression was detected in 46 patients (69.7%) and significantly reduced in poorly differentiated cancer (p=0.023). Patients with cyclin D1-expressing tumors showed better response to tamoxifen but the difference was not statistically significant. Cyclin D1 expression was associated with differentiation of the breast cancer but not useful in discriminating a good responder to tamoxifen treatment.