In this study we demonstrate the change in estrogen receptor (ER) level and cell proliferation in human breast cancer after a short-term tamoxifen therapy. Ten pre- and post-treatment breast tumor samples were examined immunohistochemically using ER and Ki-67 antibodies. Before tamoxifen treatment, six (60%) of ten patients were positive for ER. Tamoxifen increased the ER level in one patient and decreased the level in 4 patients. There was no significant change in ER level by tamoxifen therapy. On the other hand, Ki-67 labelling index (LI) significantly decreased after tamoxifen treatment. When Ki-67 LI was analyzed according to ER level, there was no difference between pre- and post-tamoxifen treatment in ER-negative patients, however, a significant decrease of Ki-67 LI by tamoxifen treatment was seen in ER-positive patients. Patients who showed down-regulation of ER expression tended to show a decrease of Ki-67 LI after tamoxifen therapy. In conclusion, short-term tamoxifen therapy decreased the proliferation of breast cancer, in ER-positive breast tumor samples.