It has recently been estimated that 43% of women between 40 and 74 have dense breasts on mammogram. That actually calculates out to 27.6 million women! By now we all know the limitations of mammography screening particularly when women have dense breasts. Not only does the increase in fibrous tissue around the ducts and lobules make it harder to distinguish a cancer, but also there are data suggesting that dense tissue itself may stimulate cancer growth. For example, studies that looked at whether tamoxifen decreased the risk of breast cancer in high-risk women suggested that the women whose breasts became less dense on the drug, which blocks estrogen, also had the most benefit in decreasing risk. Twenty-two states have passed laws requiring that you be notified if your mammogram is dense. Meanwhile websites such as DenseBreast-info.org have been developed to help women understand what dense breast tissue is and what options they might have for other forms of imaging, such as ultrasound or MRI. It is against this background that Karla Kerlikowske’s group at the University of California, San Francisco has come out with a new effort to better determine whether and to what extent breast density increases breast cancer risk. Their new study, published on May 19, 2015, in the Annals of Internal Medicine , reviewed 831,426 mammograms from 365,426 women and calculated the interval cancer rate. (Interval cancers are invasive cancers that are found within 12 months after a normal mammogram.) They then factored in breast cancer risk using the risk calculator, which calculates risk based on five factors: age, race/ethnicity, family history of breast cancer, and breast density. The study confirmed that women with dense breasts had a higher risk of breast cancer; however, they were able to show that the magnitude of risk also depended on the other risk factors assessed with the risk calculator. The chance of a cancer showing up after a “normal” mammogram (an interval cancer) was highest in women with extremely dense breasts who also had a five-year risk on the risk calculator of 2.5% or greater. For these women, learning about additional imaging with ultrasound or MRI was beneficial. But for the vast majority of women who had a low five-year risk—including those with dense breast tissue—the risk of an interval cancer was low. Another piece of good news that came out of the study is that the overall rate of false positives (mammogram shows something but on additional imaging or biopsy it turns out to be benign) is low, except for the women who are at extremely low risk and have dense breasts. These women had a higher rate of false positives. This confirms that mammographically determined breast density is only one factor in breast cancer risk, and that other factors need to be taken into consideration when women and their doctors are discussing additional screening. Figuring out who can most benefit from additional screening and who will not is an important distinction in what has become a continuing debate. The more a woman knows about her breast cancer risk, the better she will be able to make decisions about her health, including breast cancer screening.