Purpose of investigation: To evaluate the effect of tamoxifen on mammographic density using a qualitative and a semiquantitative method. Methods: Mammograms from 148 women treated for breast cancer before and after surgery were reviewed: 68 were administered tamoxifen; 80 did not receive tamoxifen. The mammograms were classified in one of the four BIRADS density categories by two radiologists blinded to the treatment and by a computer-assisted method after digitizing images. Results: At mammographic one-year-follow-up density was reduced in both groups and remained stable in the following years. A comparison of mammograms performed before surgery and after one year showed a statistically significant difference in density reduction between the tamoxifen and the non-tamoxifen-treated group. Good agreement was obtained between the qualitative and semiquantitative method. Conclusion: Breast density reduction observed in women treated with tamoxifen may help in the detection of small tumors in dense breasts by means of reducing the masking effect of parenchyma.