JNCI Vol. 102, Issue 10 | May 19, 2010 In this issue of the Journal, researchers from the Breast Cancer Surveillance Consortium (BCSC) present data on the performance of screening and diagnostic mammography in 117 738 women younger than 40 years ( 1 ). This landmark descriptive study should inform women and clinicians and guide research efforts for the early detection of breast cancer in this age group. There are a number of breast cancer screening guidelines for younger women that have been published by prominent health and medical groups including the American Cancer Society ( 2 ) and the American College of Radiology ( 3 ) that advocate screening starting at approximately age 30 years (or 10 years before the premenopausal diagnosis of breast cancer in their relevant relative) for women with BRCA1 and/or BRCA2 mutations or who are otherwise at increased risk for breast cancer. The American Cancer Society recommendation also states that “because the evidence is limited regarding the best age at which to start screening, this decision should be based on shared decision-making between patients and their health care providers, taking into account personal circumstances and preferences.” The BCSC analysis ( 1 ) provides a retrospective insight on the benefi ts and harms that might accompany such screening. It is, however, critical to point out that this article investigates mammography only and not the use of magnetic resonance imaging or ultrasound in combination with mammography in screening younger high-risk women. What would basic epidemiology principles lead us to expect to fi nd in reviewing the BCSC data, in comparison with mammography screening in women aged 40 years or older? One would expect to fi nd that, even with a specifi city similar to that in women older than 40 years, using mammography in this population with a lower prevalence of disease would result in a lower cancer detection rate, a lower positive predictive value, more false-positive results, and more callbacks for additional imaging studies and other associated workup procedures. One might expect that the known differences in breast density at younger ages could translate to differences in the screening Mammography in Younger Women: The Dilemma of Diminishing Returns