Tamoxifen is known to reduce the risk of breast cancer in women at high risk and also reduces mammographic breast density (MD) in a preventive setting. We investigated the efficacy of MD reduction (MDR) for predicting recurrence in estrogen receptor (ER)-positive patients in an adjuvant setting. A total of 1,066 ER-positive breast cancer patients who were enrolled in this study underwent curative surgery and received adjuvant tamoxifen for at least 2 years at our institution between January 2003 and December 2008. Using a computerized system, a single radiologist reviewed all mammograms and classified MD patterns on the basis of the Breast Imaging Reporting and Data System. MDR was assessed using the baseline mammogram taken before surgery (preMD) and the followup mammogram taken after the start of adjuvant tamoxifen(postMD). MDR positivity was defined as downgrading of the postMD grade, with the preMD grade as a reference.Patients were divided into 2 groups, MDR-positive and MDR-negative, for statistical analysis. Patients who showed MDR after an average of 19 months of adjuvant tamoxifen treatment had a 65 % lower risk of recurrence than patients who did not show MDR. Furthermore, significant risk reduction according to MDR had a predictive power for any type of recurrence pattern including loco-regional recurrence(87 % reduction) and systemic recurrence (52 %reduction) in ER-positive breast cancer patients, especially in women ≤50 years. In our study, only 4 patients (0.4 %)showed contralateral breast recurrence during the mean 61-month follow-up period and none of them experienced MDR. In conclusion, MDR during adjuvant tamoxifen therapy was independently associated with a lower risk of systemic and loco-regional recurrence in ER-positive breast cancer patients, especially in young women. For patients who do not experience MDR after approximately 1.5 years of tamoxifen therapy, more caution should be taken and other treatment strategies are warranted.