Purpose : To evaluate the efficacy of tamoxifen, as shown by mammograpic changes Materials and Methods : We studied the mammograms of 20 breast cancer patients treated with tamoxifen(20mg/day) and 20 patients treated with tamoxifen(20mg/day) ub combination with chemothrapy. Control groups consisted of 20 breast cancer patients treated with chemotherapy and 20 healthy women ; the patterns of age distribution and menstrual cycle among these participants were similar to these of the study groups. Two radiologists determined parenchymal changes as seen on follow-up mammogram, of the contralateral breast in patients with breast cancer, and of the left breast in healthy women. Results : Follow-up mammogram showed decreased breast parenchyma in 75% of patients treated with tamoxifen, and in 70% of patients treated with tamoxifen and chemotherapy. Mammographic changes were not noted in 85% of patients treated with chemotherapy and in 90% of healthy women. Conclusion : On follow-up mammogram, breast parenchyma was seen to have been decreased by tamoxifen, used to prevent the recurrence of breast cancer and for its antiproliferative effect. Mammography might be a suitable method for determining the effect of tamoxifen.