A recent analysis challenges earlier assumptions about how often mammography screening for breast cancer leads to overdiagnosis. Researchers from the University of Southern Denmark and other institutions reexamined data from eight major randomized screening trials and compared them with data from Denmark’s routine screening programs. Their findings suggest that overdiagnosis—when screening detects a breast cancer that would never have caused harm—may be less than 5%, rather than the previously cited estimates of 30% to 50%. This difference could have significant implications for how women and healthcare providers understand the risks and benefits of regular screening. The study highlights that timing plays a crucial role in estimating overdiagnosis. When screening is introduced, it can initially lead to more diagnoses because cancers are detected earlier. However, over time, some of these cancers would have been found anyway, even without screening. As these cases are detected sooner, the overall rate of new diagnoses may appear to decline. If a study ends before this decline becomes visible, researchers might mistakenly interpret the initial increase in diagnoses as overdiagnosis. To address this, the researchers considered various factors, such as whether women in the control group later received screening, the number of screening rounds offered, and how long participants were followed up. By accounting for these variables, the study suggests that previous high estimates of overdiagnosis may have been based on incomplete or misinterpreted data. The analysis included trials like the New York Health Insurance Plan, Malmö, Two-County, Edinburgh, the Canadian National Breast Screening Study, Stockholm, Gothenburg, and UK Age, as well as data from two regional screening programs in Denmark. The researchers examined both invasive breast cancer and ductal carcinoma in situ (DCIS), a condition where abnormal cells are found in the milk ducts. They noted that some earlier high estimates of overdiagnosis influenced screening guidelines and public communication. However, when viewed in their full temporal context, randomized trial data are consistent with overdiagnosis rates below 5%. The study aims to provide a more accurate interpretation of the evidence, helping women better understand the benefits and potential risks of undergoing regular breast cancer screening.