Previous studies have reported that tamoxifen use is associated with a decrease in mammographic breast density. This is a potentially valuable finding since mammographic sensitivity is limited by breast density. Anything that reduces breast density would theoretically enhance the sensitivity of mammography for the detection of breast cancer in women at an earlier stage when it is more curable. We performed a retrospective study investigating the potential effect of tamoxifen on breast density. The data for this retrospective study were collected from the records of 52 charts from a single medical oncologist. Patients with breast cancer were selected regardless of stage or age at the time of diagnosis or treatment, as long as their charts had records of bilateral mammograms. For each breast on each woman, both mediolateral oblique and craniocaudal views were reviewed independently by two radiologists on two separate occasions to obtain inter- and intraobserver variability. Two methods of classifying breast density were used: the Breast Imaging Reporting and Data System (BI-RADS), and measurements of percent density. Only age and menopausal status were found to be associated with breast density. There was no correlation between breast density and tamoxifen use (past or present). Our study shows no association between tamoxifen use and breast density. We confirm previous observations that breast density is inversely correlated with age and postmenopausal status.