Like all early detection strategies, screening mammography involves trade-offs. If women are to truly participate in the decision of whether or not to be screened, they need some quantification of its benefits and harms. Providing such information is a challenging task, however, given the uncertainty--and underlying professional disagreement--about the data. In this article, we attempt to bound this uncertainty by providing a range of estimates-optimistic and pessimistic--on the absolute frequency of 3 outcomes important to the mammography decision: breast cancer deaths avoided, false alarms, and overdiagnosis. Among 1000 US women aged 50 years who are screened annually for a decade, 0.3 to 3.2 will avoid a breast cancer death, 490 to 670 will have at least 1 false alarm, and 3 to 14 will be overdiagnosed and treated needlessly. We hope that these ranges help women to make a decision: either to feel comfortable about their decision to pursue screening or to feel equally comfortable about their decision not to pursue screening. For the remainder, we hope it helps start a conversation about where additional precision is most needed.