Our objective was to determine the interobserver variability of breast density assessment according to the Breast Imaging Reporting and Data System (BI-RADS) and to examine potential associations between breast density and risk factors for breast cancer. Four experienced breast radiologists received instructions regarding the use of BI-RADS and they assessed 57 mammograms into BI-RADS density categories of 1-4. The weighted kappa values for breast density between pairs of observers were 0.84 (A, B) (almost perfect agreement); 0.75 (A, C), 0.74 (A, D), 0.71 (B, C), 0.77 (B, D), 0.65 (C, D) (substantial agreement). The weighted overall kappa, measured by the intraclass correlation coefficient (ICC), was 0.77 (95% CI: 0.69-0.85). Body mass index was inversely associated with high breast density. In conclusion, overall interobserver agreement in mammographic interpretation of breast density is substantial and therefore, the BI-RADS classification for breast density is useful for standardization in a multicentre study.